FIXED: Expanded meta arguments whith specifiers greater than zero.  Now
       term_position/4 is always ground, to allow undesirable choicepoints or
       verifications for groundness, for instance in 2nd clause of extend_pos/3,
       the call to '$append'/3.  Generalized extend_pos/2 and moved from dcg.pl
       to expand.pl, which is required to do such expansion.