fact: Definition fpowerset (A : {fset K}) : {fset {fset K}} := [fset[fpowerset_key] [fsetval y in Y : {set A}] | Y in powerset [set: A]].
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)