(** ** Post Grammars *)
Fixpoint sigma (a : symbol) A :=
match A with
nil => [a]
| x/y :: A => x ++ (sigma a A) ++ y
end.
(** ** Alphabets *)
Fixpoint sym (R : SRS) :=
match R with
[] => []
| x / y :: R => x ++ y ++ sym R
end.

Lemma sym_word_l R u v  :
u / v el R -> u <<= sym R.