intros p q r H' H'0 H'1.
apply pluspf3_assoc with (l := (p, (q, r))); auto.
Qed.
Hint Resolve pluspf_assoc.

Theorem eqp_pluspf_com_l :
forall p q r,
eqP A eqA n p q ->
canonical A0 eqA ltM p ->
canonical A0 eqA ltM q ->
canonical A0 eqA ltM r -> eqP A eqA n (pluspf p r) (pluspf q r).