(* strings of natural numbers to bitstrings, [ n1, ... n2 ] |-> 1^n1 0 ... 1^n2 0 *)

(* Specifications: *)
Lemma f_s_app x y : f_s (x ++ y) = f_s x ++ f_s y.
Definition f_c '(x,y) := (f_s x, f_s y).