(* Remark - It should be possible to get rid of auxiliary compat and to combine prism0 with compat_intro in one lemma *)
(*****************************************************************)
(* Theorem prism : (U,V,W:redexes)(sub V U) ->                   *)
(*     (UV:redexes)(residuals U V UV) ->                         *)
(*     (WV:redexes)(residuals W V WV) ->                         *)
(*     (WU:redexes)(residuals W U WU) <-> (residuals WV UV WU).  *)
(*****************************************************************)

(* Converse of prism1 but needs regularity of U *)
Lemma prism2 :
forall U V W : redexes,
sub V U ->
regular U ->
forall UV : redexes,
residuals U V UV ->
forall WV : redexes,
residuals W V WV ->
forall WU : redexes, residuals WV UV WU -> residuals W U WU.