(* Mapping between two subspaces with the same dimension. *)

(* Specifications: *)
Lemma eq_rank_unitmx m1 m2 n (U : 'M_(m1, n)) (V : 'M_(m2, n)) :
\rank U = \rank V -> {f : 'M_n | f \in unitmx & V :=: U *m f}%MS.