Definition size := length (A:=Term A n).Definition sizel m := match m with
                      | (l1, l2) => size l1 + size l2
                      end.Definition lessP m1 m2 := sizel m1 < sizel m2.

Theorem pX_inj : forall (n1 n2 : Term A n) l1 l2, n1 = n2 -> l1 = l2 -> pX n1 l1 = pX n2 l2.