(* returns the proposition generated by a fact *)
Definition eval_fact i f :=
  match f with
  | eqH h1 h2 => interp i h1 = interp i h2
  | eqD d1 d2 => d1 = d2
  | eqX x1 x2 => plook i x1 = plook i x2
  end.Notation pread' x t := (odflt (dyn tt) (pread x t)).Definition pfree x t := filter (predC (ptreq x)) t.Definition hfree h t := filter (predC (vareq h)) t.

Lemma cancel_sound i t1 t2 : def (interp i t1) -> interp i t1 = interp i t2 -> eval i (cancel i t1 t2).