Definition Rminus :
  forall (a : Term A n) (nZa : ~ zeroP (A:=A) A0 eqA (n:=n) a),
  list (Term A n) -> list (Term A n) -> list (Term A n) -> list (Term A n).

Theorem minuspf_in : forall (p q : list (Term A n)) (a b : Term A n), In a p -> ltT ltM b a -> canonical A0 eqA ltM p -> canonical A0 eqA ltM (pX b q) -> In a (minuspf A A0 A1 eqA invA minusA multA eqA_dec n ltM ltM_dec p (pX b q)).