unfold spminusf in |- *.
intros a b nZb p q H' H'0 H'1; apply canonical_minuspf with (1 := cs); auto.
Qed.
Hint Resolve canonical_spminusf.

Theorem spminusf_extend :
forall (a b : Term A n) (nZb : ~ zeroP (A:=A) A0 eqA (n:=n) b)
(p q : list (Term A n)),
divP A A0 eqA multA divA n a b ->
canonical A0 eqA ltM (pX a p) ->
canonical A0 eqA ltM (pX b q) ->
eqP A eqA n (spminusf a b nZb p q) (spminusf a b nZb (pX a p) (pX b q)).