fact: Definition flattenStep {X} (x: NonemptySequence (Sequence X)) : flatten x = concatenate (flatten (composeSequence' x (dni lastelement))) (lastValue x).
imports: UniMath.Combinatorics.FiniteSets UniMath.Combinatorics.Lists UniMath.Combinatorics.Vectors UniMath.MoreFoundations.PartA UniMath.MoreFoundations.Tactics