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 Hi all, This is not a real answer, but a remark. The question was addressed in PrologIII with constraints overs the lists (and hence over the strings). An exemple is (so called Marseille syntax)     Z :: 10, <1,2,3>.Z = Z.<2,3,1> i.e :   the list Z is of the length 10 and,         the equation (constraint) to be satisfied is such that :                 <1,2,3>.Z : the list containing <1,2,3> put before Z in a list must be equal to                 Z.<2,3,1> :    the  same  Z put before the list <2,3,1>     => a solution is Z=<1,2,3,1,2,3,1,2,3,1> May be the key feature   here is the notion of concatenation (append) instead of the "cons" constructor used in Prolog std (inherited from Lisp). But as I know, PrologIII contains a special solver over what is called the monoide space (actually a solver over finite trees). Many research works was done  on the  "associative unification" with linguistic applications. I guess : adding that feature to Gprolog would be a big task. Regards Alex Jasper Taylor a écrit : Hi all, We are used to being able to create a list with a 'tail' that may contain zero or more uninstantiated elements, using syntax like L = [a, b, c | Tail]. But why should we not have a list with a section at the start or in the middle, maybe even more than one such section, containing 0 or more uninstantiated elements? If you go: L = [a, b, c | Tail], append(L, [d, e, f], L2), you get a series of solutions with Tail instantiated to different length lists. But why shouldn't you get a single solution, something like: L2 = [a,b,c, '\$opening', d,e,f], which would then unify with any list starting with [a,b,c] and finishing with [d,e,f] ? I don't see what is logically different about the end of a list from its start or middle, that a representation of an unknown number of elements should be allowed there.    --Jasper _______________________________________________ Users-prolog mailing list address@hidden http://lists.gnu.org/mailman/listinfo/users-prolog ```-- Alexandre Saidi Maitre de Conférences Ecole Centrale de Lyon-Dép. MI Tél : 0472186530, Fax : 0472186443 ```