fact: Definition map_preimg `{MapFold K A MKA, Empty MASK, PartialAlter A SK MASK, Empty SK, Singleton K SK, Union SK} (m : MKA) : MASK := map_fold (λ i, partial_alter (λ mX, Some $ {[ i ]} ∪ default ∅ mX)) ∅ m.
imports: Coq(Permutation) stdpp(options)