proposition: Theorem combine_eq_rect2 : forall sz n n' (H: n = n') H' (a: word sz) (b: word n), combine a b = eq_rect _ word (combine a (eq_rect _ word b _ H)) _ H'.
proof: Proof. induction a; simpl; intros. eq_rect_simpl; auto. erewrite WS_eq_rect. erewrite IHa. auto. Unshelve. lia. Qed.
imports: Coq.Classes.Morphisms Coq.Classes.Morphisms_Prop Coq.Classes.Equivalence Coq.Arith.PeanoNat Coq.ZArith.BinInt  Coq.Arith.Arith Coq.NArith.NArith Coq.Bool.Bool Coq.ZArith.ZArith Coq.Logic.Eqdep_dec Coq.Logic.EqdepFacts Coq.Program.Tactics Coq.Program.Equality Coq.setoid_ring.Ring Coq.setoid_ring.Ring_polynom bbv.Nomega bbv.ZLib bbv.NatLib bbv.NLib bbv.N_Z_nat_conversions bbv.DepEq bbv.DepEqNat bbv.ReservedNotations Coq.micromega.Lia