(** * SRH to SR *)
Section SRH_SR.
Variables (R : SRS) (x0 : string) (a0 : symbol).
Notation Sigma :=  (a0 :: x0 ++ sym R).
Definition P :=
R ++ map (fun a => [a; a0] / [a0]) Sigma ++ map (fun a => [a0; a] / [a0]) Sigma.

Lemma x_rewt_a0 x :
a0 el x -> x <<= Sigma -> rewt P x [a0].