Existence of an identity element for addition

nat : type.
z : nat.
s : {x:nat} nat.

plus : {N1:nat}{N2:nat}{N3:nat} type.
plus_z : {N:nat} plus z N N.
plus_s : {N1:nat}{N2:nat}{N3:nat}
  {P:plus N1 N2 N3} plus (s N1) N2 (s N3).

Specification "ident.lf".

Theorem identity : exists I, ((forall X, {X:nat} => exists D, {D:plus I X X}) /\
                              (forall X, {X:nat} => exists D, {D:plus X I X})).
exists z. split.
  %left identity
    intros. exists (plus_z X). search.
  %right identity
    induction on 1. intros. case H1.
      %N=(s x)
        apply IH to H2. exists (plus_s x z x D). search.
      %N=z
        exists (plus_z z). search.