(** The two following lemmas come from Theorem 22.4 *)

(* Specifications: *)
Lemma cong2_lam2__cong : forall N M P Q N' M' P' Q',
Lambert N M P Q -> Lambert N' M' P' Q' ->
Cong N Q N' Q' -> Cong P Q P' Q' ->
Cong N M N' M'.
Lemma cong2_lam2__conga : forall N M P Q N' M' P' Q',
Lambert N M P Q -> Lambert N' M' P' Q' ->
Cong N Q N' Q' -> Cong P Q P' Q' ->
CongA M P Q M' P' Q'.
Lemma cong2_sac2__cong : forall A B C D A' B' C' D',
Saccheri A B C D -> Saccheri A' B' C' D' ->
Cong A B A' B' -> Cong A D A' D' ->
Cong B C B' C'.
Lemma sac__perp1214 : forall A B C D, Saccheri A B C D -> Perp A B A D.
Lemma sac__perp3414 : forall A B C D, Saccheri A B C D -> Perp C D A D.
Lemma cop_sac2__sac : forall A B C D E F,
Saccheri A B C D -> Saccheri A B E F -> D<>F -> Coplanar A B D F -> Saccheri D C E F.