Fixpoint convI (e : BC) : BCI :=
  match e with
    | zero => zeroI
    | proj n s i =>
      if leb (S i) n then projIn i else projIs (i - n)
    | succ b => succI b
    | pred => predI
    | cond => condI
    | rec g h0 h1 => recI (convI g) (convI h0) (convI h1)
    | comp n s g ln ls =>
      compI (convI g) (map convI ln) (map convI ls)
  end.

Lemma convI_correct : forall e n s, arities e = ok_arities n s -> forall vnl vsl, semI (convI e) vnl vsl = sem e vnl vsl.