:list:member declares ArgModeOutput but Decide handles a bound first argument #94

Summary

:list:member's declared mode is narrower than its implementation. The analyser refuses a bound
first argument that Decide would have handled, so a membership test is rejected while the code
to perform it is present and reachable.

Where

builtin/builtin.go, both on main at the time of writing:

// line 54 — the declaration
symbols.ListMember: {ast.ArgModeOutput, ast.ArgModeInput},

// line ~445 — the evaluator, which handles the bound case
evaluatedMember := atom.Args[0]
memberVar, memberIsVar := evaluatedMember.(ast.Variable)
if memberIsVar && subst != nil {
    evaluatedMember = subst.Get(memberVar)
    _, memberIsVar = evaluatedMember.(ast.Variable)
}
if !memberIsVar { // We are looking for a member
    res, err := functional.EvalExpr(
        ast.ApplyFn{symbols.ListContains, []ast.BaseTerm{evaluatedArg, evaluatedMember}}, nil)
    ...
}

The if !memberIsVar branch and its comment are the bound case, delegating to :list:contains.
ArgModeOutput means analysis never lets a program reach it.

Reproduction

Against unpatched main:

seen(/a).
ok(X) :- seen(X), :list:member(X, [/a, /b]).

ANALYSIS ERROR: for goal ":list:member(X,fn:list(/a,/b))" expected X (arg 0) to be a free variable

Four shapes tried; three are refused:

The negated form being accepted is worth noting: my first attempt at a reproduction used it, found
nothing, and would have had me conclude there was no issue.

Changing the declaration to {ast.ArgModeInputOutput, ast.ArgModeInput} accepts all four, and the
existing builtin tests pass unchanged.

What I am not proposing

I am not sending a patch. We carried that one-token change in a fork and have now removed it,
because we do not use the capability — our one call site uses the generator form, which needs no
change. Reporting it rather than patching it seemed the right way round.

Two things I have not established, and would defer to you on:

• whether the restriction is deliberate, with the evaluator branch kept for a caller I have not
  found; and
• whether InputOutput is the right widening, as against a separate predicate.

Filed here rather than on Codeberg because I have no account there; happy to move it.

Context: this is the same reporter as #92 and #93. Both of those rested on claims I could not
reproduce and I have withdrawn #93 accordingly. This one I did reproduce, and the shapes and the
error text above are what that looks like — it seemed only fair to show the work this time.