Qed.
Hint Resolve p0_pluspf_l p0_pluspf_r.

Theorem plusTerm_is_pX :
forall (a : Term A n) (p : list (Term A n)),
canonical A0 eqA ltM (pX a p) ->
eqP A eqA n (pluspf (pX a (pO A n)) p) (pX a p).