pre-table: changed how sub-string assignments get sub-string code

changed debug names of sub-string assignment codes to include base name
removed sub-string as second associated code from sub-string assignments
use name to look up base sub-string code (not second associated code)
use multiple flag to identify when to use next base sub-string code
added multiple flag to mid 3 assignment codes
changed table check to ignore multiple flag when reference flag set
added keep table flag to identify string assignment keep codes