Goal:
  
  A : Type
  B : Type
  C : Type
  f : option A -> option B -> option C
  f_None_None : f None None = None
  ============================
   (forall (m1 : tree' A) (m2 : tree' B) (i : positive),
    get i (combine'_direct m1 m2) = f (get' i m1) (get' i m2))


DEBUG:Going to execute:
induction m1; destruct m2