fact: Fixpoint flatten `(xs : list (list A)) : list A := match xs with | nil => nil | cons x xs' => app x (flatten xs') end.
imports: Coq.Lists.List Equations(Equations) Equations.Type.EqDec Category.Lib Category.Theory.Coq.Functor Category.Theory.Coq.Applicative Category.Theory.Coq.Monad Category.Theory.Coq.Maybe Coq.Lists.List