The generic "repeatedly recompute a value from itself until it stops changing" primitive -- extracted from a pattern that had already been hand-written three separate times in Ichor's own compiler internals before this existed: Grammar.Analysis's private fixpoint/3 (nullable-set and always-empty-set computation, both over a MapSet.t(atom()) of rule/token names) and Grammar.LRTable.Sets' private fixpoint/2/fixpoint_map/2 (FIRST/FOLLOW-set computation, one over a MapSet, one over a Map). None of those three needed anything MapSet- or Map-specific about the iteration itself -- only about what step_fn does with the value in between -- which is exactly what this generalizes: step_fn closes over whatever fixed data it needs (a grammar's own rule list, a graph, ...), this module never sees that data at all, only the value being iterated.

A "fixpoint" here always means a reached, not merely a theoretical, one: least/2 recurses until step_fn produces a value structurally equal (==/2) to its input, so step_fn must be monotone (each step only grows/refines the value, never oscillates) for this to actually terminate -- the same requirement every one of the three call sites above already relies on implicitly (each only ever adds names to a set, never removes any).