fact: Definition flattenUnorderedSequence {X : UU} : UnorderedSequence (UnorderedSequence X) -> UnorderedSequence X.
imports: UniMath.Combinatorics.FiniteSets UniMath.Combinatorics.Lists UniMath.Combinatorics.Vectors UniMath.MoreFoundations.PartA UniMath.MoreFoundations.Tactics