Section Recr.
(** [recr] satisfies the fixpoint equation used for its definition. *)
Variable (A:Type)(case0:A)(caserec:digits->int31->A->A).

Lemma recr_eqn : forall x, iszero x = false ->
recr A case0 caserec x =
caserec (firstr x) (shiftr x) (recr A case0 caserec (shiftr x)).