Abstract:
A system and a method are provided employing the concept of Budget Queue Time to define the priority of lots while distinguishing clearly between the controllable an uncontrollable portions of the remaining production time needed and to make the priority setting further meet the actual status. Two indices X and P are used concurrently to define the priority of a lot. X is the index of the delivery week which indicates the week in which the lot must be out of the fabrication process and P denotes the temporary priority according to the Budget Queue Time, but X is the dominant one of the two indices X and P. Use is made of the concept of remaining Budget Queue Time instead of traditional queue time of current stage for dispatching to reduce the variance of cycle time variance.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to allocation of priority of lots of work to processing in a factory and more particularly to ranking of lots to determine their priority. 
     2. Description of Related Art 
     U.S. Pat. No. 5,319,544 of Schmerer et al. for “Computerized Inventory Monitoring and Verification System and Method”; U.S. Pat. No. 5,128,861 of Kagami et al. for “Inventory Control Method and System”; and U.S. Pat. No. 5,396,432 of Saka et al. for “Versatile Production System and Method of Operating Same” show production systems. 
     U.S. Pat. No. 5,446,671 of Weaver et al. for “Look-Ahead Method for Maintaining Optimum Queued Quantities of In-Process Parts at a Manufacturing Bottleneck” shows a production system. 
     U.S. Pat. No. 5,546,326 of Tai et al. for “Dynamic Dispatching Rule that Uses Long Term Due Date and Short Term Queue Time to Improve Delivery Performance”; U.S. Pat. No. 5,612,866 of Weng for “Method and System for Dynamic Dispatching in Semiconductor Manufacturing Plants”; and U.S. Pat. No. 5,721,686 of Shahraray et al. for “Method and Apparatus for Control and Evaluation of Jobs in a Factory” show scheduling systems. 
     SUMMARY OF THE INVENTION 
     GLOSSARY 
     Budget_Q . . . Budget Queue Time or BQT 
     BR . . . Base Rate 
     Critical Ratio.C/R: Critical Ratio Degree of Customer Dissatisfaction 
     C tR  . . . BT (Budget Time) according to Required Date of lot 
     C tD  . . . BT (Budget Time) by the Due Date of lot 
     C tqR  . . . Budget_Q (BQT) according to Required Date of lot 
     C tqD  . . . Budget_Q (BQT) by Due Date of lot 
     C tI  . . . Step Budget_Q (BQT) for Lot Lm, Ln, etc. 
     CT . . . Cycle Time 
     Due_Date . . . Scheduled Due Date from MPS, i.e. date when processing of a lot must be finished for an order placed on MPS. 
     Due_Date_Now . . . Current Scheduled Due Date 
     FAB . . . Fabrication plant 
     FFOT . . . Forecast_FAB_Out_Time 
     Finished out . . . Completed the manufacturing process 
     MTD . . . Month To Date 
     MTD_OUT . . . Month To Date wafers OUT of FAB 
     MPS . . . Master Production Schedule 
     OTD . . . On-Time-Delivery 
     P . . . Temporary priority (P value) according to BQT 
     RP . . . Remaining Theoretical Recipe Processing Time 
     RD . . . (Required Date) Date when a lot must be finished out according to start date and average past cycle time data. 
     SLACK . . . Due_Date_Now—FFOT 
     Least SLACK . . . Lot with least amount of SLACK 
     t p  . . . time from current step to finished step 
     t sp  . . . Process time from step s from to p (process): Recipe time (process time) by step by product 
     Σt sp  . . . Remaining theoretical process time equal to sum of t p  from current step to finished step. 
     T . . . Time 
     WIP . . . Work in Process 
     X . . . Index for Week when lot must be Delivered i.e. Lot is Required to be out of FAB 
     The required cycle time is based on PC Lot&#39;s RD definition based on the FIFO principle. 
     PROBLEM SOLVED BY INVENTION 
     Two of the indices most commonly used for setting lot priorities are least SLACK and the critical ratio (C/R). These two indices are used to dispatch with the aim of meeting or improving OTD. However, the values of SLACK or C/R sometimes cannot reflect the actual urgent status of a lot due to the variances of the uncontrollable portion of the remaining Cycle Time (CT), i.e. the remaining recipe processing time between different products. Thus, only after the processing time is deducted while calculating the remaining time, is it possible to define the urgency or the priority of lots clearly. There are two kinds of dates/times when a lot should have completed processing, which are the Required Date (RD) and the Due_Date. The RD denotes the delivery requirement and the Due_Date indicates the shorter cycle time deviation. In practice, the two values, i.e. (the RD and the Due_Date) can be combined together and integrated into a date for calculating the remaining CT. While the combination of the two dates is a convenience; it can lead an operator to be confused by the two different concepts of delivery time and CT variance when evaluating the priority to be assigned to a given lot. 
     Problems solved and improvements obtained by this invention include the features as follows: 
     1. With the index of X, the lots are pushed more correctly for delivery especially for the back end stages. 
     2. With the index of P, the lots are moved according to the remaining Budget Queue Time. This makes lots finished out with little variance. 
     3. With the distinguishability between delivery and cycle time variance of lots by two parameters of (X,P), the analysis of delivery and variance of the whole line is made easily. Production planners benefit greatly as they are able to make scheduling decisions resolving conflicts arising upon conflicting customer requirements. 
     The concept of Budget Queue Time is used to define the priority of lots while distinguishing clearly between the controllable an uncontrollable portions of the remaining production time needed and to make the priority setting further meet the actual status. Two indices X and P are used concurrently to define the priority of a lot. X is the index of the delivery week which indicates the week in which the lot must be out of the fabrication process and P denotes the temporary priority according to the Budget Queue Time, but X is the dominant one of the two indices X and P. Use is made of the concept of remaining Budget Queue Time instead of traditional queue time of current stage for dispatching to reduce the variance of cycle time variance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other aspects and advantages of this invention are explained and described below with reference to the accompanying drawings, in which: 
     FIGS. 1,  2 ,  3  and  4  are computer flow charts showing the steps of a method of operation of a production control system in accordance with the method and apparatus this invention. 
     FIG. 5 shows a manufacturing plant including a central computer system and a fabrication plant which includes a computer system for allocating plant resources using budget queue time measurements for allocating priorities to lots of work to be processed in accordance with this invention. 
     FIG. 6 is an example of two lots being processed according to the method of this invention. In the case of one lot, the processing speed of the lot is increased and in the case of another lot the processing speed of the lot is slowed down. 
     FIG. 7A shows a table for four stages and four different lots of the same product in different stages. 
     FIG. 7B shows a table related to the table of FIG. 7A for the four stages and four different lots of the same product in different stages indicating the values of the minimum number of days and the delivery week X. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1,  2 ,  3  and  4  are flow charts showing the steps of a method of operation of a production control system in accordance with the method and apparatus this invention. The computer programs of FIGS. 1,  2 ,  3  and  4  in accordance with this invention are resident in a fabrication plant computer system  70  which is shown in FIG. 5 connected as a part of the overall computer system  50 . FIG. 5 shows a manufacturing plant  50  including a central computer system  60  and a fabrication plant  90  which includes a computer system  70  for allocating plant resources using budget queue time measurements for allocating priorities to lots of work to be processed in accordance with this invention. 
     1. Calculation of Budget Q for Lots Lm, Ln, etc. at Each Step 
     FIG. 1 is a flow chart in block diagram form with steps shown in separate blocks illustrating a method of operating the computer system  50  of FIG. 5 to calculate the Budget_Q for all Lots Lm, Ln, etc. included in the in-line WIP at each step. 
     The first three blocks involve definition and determination of data being supplied or which has been supplied to the CPU  71 . Those sets of data are stored by the system  70  in data input blocks  11 A,  11 B and  11 C after definition and determination of the dates and time as described below. 
     In two of those blocks required in order to calculate the Budget Queue Time and to consider the delivery and cycle time variance also, it is required, as a preliminary matter, to define the RD when a lot Lm, Ln, etc. is completed (finished out) in block  11 A and the due dates of lots Lm, Ln, etc. in block  11 B, as indicated by FIG.  1 . 
     In block  11 A, the RD&#39;s of the lots Lm, Ln, etc. to be processed by plant  90  are defined. The RD&#39;s are the dates when a lot must be “finished out” according to the start date and the average cycle time data of the past. 
     In block  11 B, data defining the Due_Dates of the lots Lm, Ln, etc. to be processed by plant  90  are defined. Due_Dates are dates scheduled from the Master Production Schedule (MPS), i.e. the date when processing of one of the lots Lm, Ln, etc. must be finished in an order placed on the MPS. 
     In block  11 C, the recipe time “t sp ” is determined for the process steps to be performed by plant  90 . The processing time of each step for each product is derived by the data determined from the production personnel and is defined as: 
     t sp =Recipe time (process time), where 
     s=step and p=process 
     The total Budget Queue Time for all remaining processing steps of a lot is calculated according the RD and due date individually and by deducting the remaining processing time (the uncontrollable portion of cycle time) from the remaining determined cycle time. And those parameters are defined as follows: 
     C tR =step Budget Time according to the RD of lot. 
     C tD =step Budget Time according to the Due Date of Lot. 
     t p  is the time from the current step to the finished step. 
     Σt sp  is the remaining theoretical process time. 
     The above parameters are calculated in accordance with the equations as follows: 
     1.1 t p =Σ t sp  Remaining theoretical process time= 
     1.2 C tR =(RD date−current date−tp) 
     1.3 C tD =(Due_Date−current date−tp) and defined as follows: 
     C tqR  is the step budget_Q time by the RD. 
     C tqD  is the step budget_Q time by the Due_Date. 
     C tI  is the step budget_Q time of the lot Lm, Ln, etc.                C   tqR     =       C   tR     ×       t   sp       t   p               1.4               C   tqD     =       C   tD     ×       t   sp       t   p               1.5                              
     1.6 C tI =min (C tqR , C tqD ) 
     2. Compare Budget_Q for the Lot Lm for the Same Product at Different Stages and through the Budget_Q to Determine the Index X where X representing the week when the lot is required to be delivered, i.e. out of the FAB by an integer 
     One of the CPUs (Central Processing Units)  61  or  71  is selected by the user to perform the function of defining the Integer X based on a comparison of Budget_Q with RP (Remaining Theoretical Recipe Processing Time) for a lot. 
     2.1 If lot&#39;s budget_Q&gt;1.1*RP, then X is determined by MPS. 
     2.2 If lot&#39;s budget_Q &lt;1.1*RP, then X is determined by the integer I where        I   =     RP     7                 days                              
     3. Setting Temporary Priority P According to Budget Queue Time 
     FIG. 3 is a flow chart in block diagram form with steps shown in separate blocks illustrating a method of operating the computer system  50  of FIG. 5 to set the temporary priority P value using only the Budget Queue Time as a measurement. The method of operating the computer system  50  only according to Budget Queue Time involves the steps, starting with block  30 , as follows: 
     3.1. A test is made in decision block  31  as to whether Budget_Q&lt;1.5*BR. 
     If the result of the test in decision block  31 , is YES, then the system proceeds to block  32  in which the priority value P for the lot is set as P=1. Then the system proceeds along lines  33  and  34  to END block  44 , which ends the program. 
     If the result of the test in decision block  31  is NO, then the system proceeds along line  31 N to decision block  34 . 
     3.2. The next test is made in decision block  34  as to whether Budget_Q&lt;2*BR. 
     3.2.1 If the result of the test in decision block  34  is YES, then the system proceeds to block  35  in which the priority value P for the lot is set as P=2. Then the system proceeds along line  36  to END block  44 , which ends the program. 
     3.2.2 If the result of the test in decision block  34  is NO, then the system proceeds along line  34 N to decision block  37 . 
     3.3. The next test is made in decision block  34  as to whether Budget_Q&lt;3*BR. 
     3.3.1 If the result of the test in decision block  37 , is YES, then the system proceeds to block  38  in which the priority value P for the lot is set as P=3. Then the system proceeds along lines  39  and  34  to END block  44 , which ends the program. 
     3.3.2 If the result of the test in decision block  37  is NO, then the system proceeds along line  37 N to decision block  40 . 
     3.4 The next test is made in decision block  40  as to whether Budget_Q&lt;4*BR. 
     3.4.1 If the result of the test in decision block  40  is YES, then the system proceeds to block  41  in which the priority value P for the lot is set as P=4. Then the system proceeds along lines  42  and  36  to END block  44 , which ends the program. 
     3.5 If the result of the test in decision block  34  is NO, then the system proceeds along line  40 N to block  37  in which the priority value P for the lot is set as P=5. 
     Then the system proceeds along lines  42  and  36  to END block  44 , which ends the program. 
     All lots which have priority values P of 1, 2, or 3 will meet the requirements as follows: 
     a. Normal production lots 1.9 days per lot; 
     b. Super hot lots 0.7 days per lot; 
     c. Hot lot or/Pilot lot 0.9 days per lot. 
     4. Dispatching Rules 
     FIG. 4 is a flow chart in block diagram form with steps shown in separate blocks illustrates a method of operating the computer system  50  of FIG. 5 for dispatching when lots Lm and Ln which are being processed at the same time, i.e. simultaneously, are in competition for the same capacity. In accordance with this invention, the processing priorities are set according to a set of rules. The method of operating the computer system  50  for dispatching involves the steps, starting with block  46 , as follows: 
     4.1. A test is made in decision block  47  as to whether the X values (Index of the Delivery Week in which the lot must be out of the FAB) of lots Lm and Ln are different. 
     If the result of the test in decision block  47 , is NO, then the system proceeds along line  47 N to block  48  in which the a higher priority “P” is given to the Lot Lm or Ln with the smaller value of X. 
     Then, the system proceeds along lines  48 ′ to END block  50 , which ends the dispatching rules program for the lots Lm and Ln. 
     4.2 If on the other hand the result of the test in decision block  47  is YES the X values of lots Lm and Ln are equal, then the system proceeds along line  47 Y to block  48  which gives a higher priority P to Lot Lm or Ln with the higher value P (temporary value P). Then, the system proceeds along lines  49 ′ to END block  50 , which ends the dispatching rules program for the lots Lm and Ln. 
     5. Effect and Improvement of Reducing Cycle Time Variance 
     Use the new Budget_Q instead of conventional q-time that will induce a convergence result with little variance of delivery time. 
     Referring again to FIG. 5, computer system  70  operates as a part of a fabrication plant  90  and so it is shown located within the plant  90 , but it may be located elsewhere as will be obvious to those skilled in the art and it can be a portion of an overall consolidated system incorporating the central system  60  and can operate independently as a matter of choice. 
     FIG. 5 also shows an overall computer system  50  employed in a manufacturing environment. System  50  includes a central computer system  60  which comprises a CPU (Central Processing Unit)  61 , a terminal  67  with monitor  62  and keyboard  63  connected to the CPU  61  for receiving/sending data respectively from/to the CPU  61 . A RAM (Random Access Memory)  65  and a DASD  64  associated with the CPU  61  are shown connected for bidirectional communication of data to and from CPU  61 . 
     Lines  76 ,  176  and  276  interconnect the CPU  61  and the CPU  71  of a fabrication plant computer system  70 . Line  176  connects between lines  76  and  276  at the interfaces of computer  60  and a factory control computer system  70  respectively. 
     The factory control computer system  70  comprises a CPU  71 , a terminal  77  with monitor  72  and keyboard  73  connected to the CPU  71  for receiving/sending data respectively from/to the CPU  71 . A random access memory  75  and a DASD  74  associated with the CPU  71  are shown connected for bidirectional communication of data to and from CPU  71 . 
     Line  86  connects from CPU  71  to line  186  connects through the factory control computer  70  interface to the shop floor system  87 . 
     The system  50  includes the data defining the Master Production Schedule for the plant  90  stored in a conventional manner, as will be well understood by those skilled in the art. 
     FIG. 6 illustrates an example of two lots being processed in accordance with the method of this invention. In the case of one lot, the speed of processing that lot is increased and in the case of another lot the speed of processing that lot is slowed down. 
     In particular, referring again to FIG. 6, in the case of Lot 1, the speed of processing that lot is increased and in the case of Lot 2 the speed of processing that Lot 2 is slowed down. 
     6. Critical Difference and Benefit Compared with the Past “Budget_Q” is used to produce “Lot XP” and to use the delivery week—X instead of conventional “Lot P” to narrow the focus on the right lot to be delivered at the same stage. 
     A lot with a scheduled delivery week in week one (1) and with a priority of five (5) should be run faster than a lot with a scheduled delivery week in week two (2) with priority of one (1) at the same stage. Through use of this concept, the capacity will be used exactly to the maximum. 
     A “Budget_Q is provided for each process step in place of the conventional step Q_time to become a “dispatching driver” which can provide a convergence result of both dispatching lots and making a critical difference in “On-time-delivery” (OTD) with a little variance in production cycle time CT among all wafers passing out of the fabrication facility. 
     EXAMPLE 
     Modeling for a Single Product 
     Referring to FIGS. 7A and 7B, to define the delivery week for a lot a chart is shown for the same product at different stages as indicated in the table below, as well. Referring to both FIGS. 7A and 7B the assumptions are as follows: 
     1. Size of Lot=24 pieces 
     2. Output Target=72 pieces 
     3. Exclude all hold lots 
     The table shown in FIG. 7A shows four stages from stage 1 to stage 4 with lot 1 at stage 1, lot 3 at stage 2, lot 2 at stage 3 and lot 4 at stage 4. 
     a. For Stage 1, lot 1 the cycle time (CT) is 4 days and the process time (T) is 0.2 days. 
     b. For Stage 2, lot 3 the cycle time (CT) is 3 days and again the process time (T) is 0.2 days. 
     c. For Stage 3, lot 2 the cycle time (CT) is 2 days and once again, the process time (T) is 0.2 days. 
     d. For Stage 4, lot 4 the cycle time (CT) is only 1 day and the process time (T) remains as 0.2 days 
     CT: Cycle Time 
     Process T: Processing Time 
     (Month To Date) MTD_OUT 24 1 day(CT) (0.2 Process T) 
     The customer would need Lot 4 and Lot 2 to get the required quantity 72 pieces, but the customer would complain about the variance between the cycle time CT1 of Lot 1 and cycle time CT4 of Lot 4, unless the present invention can make up “SLACK” of Lot 1. 
     FIG. 7B shows a table defining the delivery week of a lot is shown for four exemplary lots of the same product for the four stages (1-4) and four different lots of the same product in different stages indicating the values of the minimum number of days and the delivery week X. 
     1 Lot Budget_Q=Min (Budget_Qr, Budget_Qd) 
     2 D=Integer (Remaining theoretical Process Time/7 (days)) 
     3 If Lot Budget_Q&lt;1.1 x Budget 
     4 Modeling for Multiple products at the same Stage 
     5 To Define the lots&#39; priority (Excluding all held lots) 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                 Remaining 
                 Re- 
                 1.5 
                 2× 
                 2.5 
                 3× 
               
               
                   
                 Process 
                 main- 
                 Budget —   
                 Budget 
                 Budget 
                 Budget —   
               
               
                 Pro- 
                 time 
                 ing 
                 Q_time 
                 Q_time 
                 Q_time 
                 Q_time 
               
               
                 duct 
                 (days) 
                 Steps 
                 (hrs) 
                 (hrs) 
                 (hrs) 
                 (hrs) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 P1 
                 12.2 
                 148 
                 0.99 
                 1.98 
                 2.97 
                 3.96 
               
               
                 P2 
                 11.3 
                 142 
                 0.95 
                 1.91 
                 2.86 
                 3.82 
               
               
                 P3 
                 13 
                 156 
                 1.00 
                 2.00 
                 3.00 
                 4.00 
               
               
                 P4 
                 14.1 
                 168 
                 1.01 
                 2.01 
                 3.02 
                 4.03 
               
               
                   
               
             
          
         
       
     
     
       
         
           
             
               
                 
                   
                     Required 
                      
                     
                         
                     
                      
                     
                       Budget 
                       — 
                     
                      
                     Q 
                   
                   = 
                   
                       
                   
                    
                   
                     ( 
                     
                       Remaining 
                        
                       
                           
                       
                        
                       days 
                     
                     ) 
                   
                 
               
             
             
               
                 
                   = 
                   
                       
                   
                    
                   
                     
                       
                         
                           
                             
                               
                                 Req 
                                 — 
                               
                                
                               date 
                             
                             - 
                             Today 
                           
                           ) 
                         
                         - 
                         
                           Remaining 
                            
                           
                               
                           
                            
                           Process 
                            
                           
                               
                           
                            
                           time 
                         
                       
                       ) 
                     
                     
                       Remaining 
                        
                       
                           
                       
                        
                       Steps 
                     
                   
                 
               
             
             
               
                 
                   
                     Due 
                      
                     
                         
                     
                      
                     
                       Budget 
                       — 
                     
                      
                     Q 
                   
                   = 
                   
                       
                   
                    
                   
                     ( 
                     
                       Remaining 
                        
                       
                           
                       
                        
                       days 
                     
                     ) 
                   
                 
               
             
             
               
                 
                   = 
                   
                       
                   
                    
                   
                     
                       
                         ( 
                         
                           
                             
                               Due 
                               — 
                             
                              
                             date 
                           
                           - 
                           Today 
                         
                         ) 
                       
                       - 
                       
                         Remaining 
                          
                         
                             
                         
                          
                         Process 
                          
                         
                             
                         
                          
                         time 
                       
                     
                     
                       Remaining 
                        
                       
                           
                       
                        
                       Steps 
                     
                   
                 
               
             
           
         
                 
         
             
         
      
     
     Min Px (Required Budget_Q, Due Budget_Q) =Px Budget_Q 
     Modeling for Multiple products at the same Stage 
     Assumption: Normal production lot 1.9 days per layer 
     Fab 6 1st lot 0.35 days per layer (Priority 1) 
     Super hot lot 0.7 days per layer (Priority 1 
     Hot/Pilot lot 0.9 days per layer (Priority 1 
     Priority settings will focus on “Normal production lots” and exclude all held lots. 
     Priority Distribution is Based on the Budget_Q of each lot under different levels of X. 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 Theoretical Process time 
               
             
          
           
               
                   
                 Priority 
               
             
          
           
               
                   
                 Budget_Q (C t1 ) 
                 1 
                 2 
                 3 
                 4 
                 5 
               
               
                   
                   
               
               
                   
                 &lt;1.5X 
                 1 
                   
                   
                   
                   
               
               
                   
                 1.5X &lt; CT &lt; 2.5X 
                   
                 2 
               
               
                   
                 2.5X &lt; CT &lt; 3X   
                   
                   
                 3 
               
               
                   
                 3X &lt; CT &lt; 4X 
                   
                   
                   
                 4 
               
               
                   
                 &gt;4X   
                   
                   
                   
                   
                 5 
               
               
                   
                   
               
             
          
         
       
     
     LOT XP 
     Definition of the Delivery Week X 
     When the normal lots are selected with over 
     1.1* (theoretical Budget_Q), then follow the PC MPS schedule to define the value of delivery week X for the lots. 
     If the delivery week for the lots is less than 1.1*Budget_Q, then in accordance with this invention, the delivery week X is redefined.        D   =       Integer                   (     Remaining                 theoretical                 process                 time     )         7                   (   days   )                                              t   sp     =                Recipe                 time                   (     process                 time     )                                s        :                   step                              p        :                   process                                   
     The time t sp  is by step. 
     C tR =Required cycle time (based on PC Lot&#39;s required date definition—FIFO principle) 
     C td =Due-date cycle time 
     This is based on actual Lot&#39;s due-date to meet MPS volume demand.                t   p     =                ∑     t   sp                                  t   sp          :                   Remaining                 theoretical                 process                 time                                t   p          :                   process                 time                 from                 current                 step                 to                 finished                 step                                C   tqR          :                     Budget   —        Q                 time                 by                   required-date                     C   tqR     =                    C   tR     ×     t   sp         t   p                                  C   tqD          :                     Budget   —        Q                 time                 by                   due   —        date                   C   tqD     =                  C     td   ×   t                 sp         t   p                                    
     C tI : Lot&#39;s step Budget_Q-time 
     C tI =min (C tqR , C tqD ) 
     C tI  is to be used as a priority definition factor of a lot for table III above. 
     While this invention has been described in terms of the above specific embodiment(s), those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the appended claims, i.e. that changes can be made in form and detail, without departing from the spirit and scope of the invention. Accordingly all such changes come within the purview of the present invention and the invention encompasses the subject matter of the claims which follow.