Module Solver_mono.UnhintSource
Unhinted mode is similar to ('a, 'd) mode, but its several outermost morphism applications are unhinted.
Treat a regular mode as an unhinted mode, by taking the identity morphism as the outermost unhinted morphism.
Takes an unhinted mode, annotate the outermost unhinted morphisms (as a whole) with the given hint, which gives a regular mode.