Fix setof/3 duplicate grouping for variant witnesses (issue #3151)

[jjtolton]

Summary

Fixes #3151 - setof/3 was creating duplicate groups for variant witnesses

Problem

When using setof/3 with witness variables, variants with the same structure were being grouped separately instead of together.

Test case:

p([_A,_A,_B]).
p([_A,_B,_A]).
p([_A,_A,_B]).

?- countall(p(_),M), countall(setof(t,p(L),_),N), write((M,N)), nl.

Expected: 3,2 (3 clauses, 2 unique variant groups)
Actual (before fix): 3,3 (incorrectly counted 3 groups)

Clauses 1 and 3 have the same variant structure [X,X,Y] and should be grouped together, but were being counted as separate groups.

Root Cause

The group_by_variant/4 predicate in src/lib/builtins.pl only grouped consecutive variants after sorting with $keysort_with_constant_var_ordering/2.

With VarComparison::Indistinct, all variable terms compare as equal and the sort is stable (maintains original order). This means variants with the same structure like [X,X,Y] from clauses 1 and 3 can be separated by different structures like [X,Y,X] from clause 2 in the sorted list.

The original implementation only checked the next element in the list, so it would miss variants that appeared later.

Solution

Modified group_by_variant/4 (src/lib/builtins.pl:993-1001) to recursively scan the ENTIRE list for ALL variants, not just consecutive elements.

Tail-recursive implementation:

group_by_variant([V2-S2 | Pairs], V1-S1, Solutions, Pairs0) :-
    (  variant(V1, V2) ->
       V1 = V2,
       Solutions = [S2 | MoreSolutions],
       group_by_variant(Pairs, V1-S1, MoreSolutions, Pairs0)
    ;  Pairs0 = [V2-S2 | RestPairs],
       group_by_variant(Pairs, V1-S1, Solutions, RestPairs)
    ).
group_by_variant([], _, [], []).

The key insight: in the else branch, we unify Pairs0 with [V2-S2 | RestPairs] before the recursive call, making it tail-recursive.

Test Plan

• Added failing test following TDD methodology (tests/scryer/cli/issues/issue_3151.*)
• Implemented fix (TDD Green phase)
• Verified test now passes with correct output 3,2
• Made predicate tail-recursive for better performance
• All existing tests still passing

Commits

1. 7de9d0d4 Add failing test for setof/3 duplicate grouping bug
2. 5a204fe7 Fix setof/3 duplicate grouping bug (issue setof/3 problem #3151)
3. 86ba395c Make group_by_variant/4 tail-recursive

• J.J.'s Robot

[jjtolton]

Test Results

No CI - ran locally:

• ✅ Issue setof/3 problem #3151 test passes (correct output: 3,2)

• ✅ All 36 existing tests pass

• ✅ No regressions

• J.J.'s Robot