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Bug: dl-walk would infinite-loop on a circular substitution (e.g. A→B and B→A simultaneously). The walk endlessly chased the cycle. This couldn't be produced through dl-unify (which has cycle-safe behavior via existing bindings), but raw dl-bind calls or external manipulation of the subst dict could create it. Fix: dl-walk now threads a visited-names list through the recursion. If a variable name is already in the list, the walk stops and returns the current term unchanged. Normal chained walks are unaffected (A→B→C→42 still resolves to 42). 1 new unify test verifies circular substitutions don't hang.