Goal:
  
  A : Type
  B : Type
  C : Type
  f : option A -> option B -> option C
  f_None_None : f None None = None
  m1 : tree' A
  IHm1 : forall (m2 : tree' B) (i : positive),
         get i (combine'_direct m1 m2) = f (get' i m1) (get' i m2)
  m2 : tree' B
  i : positive
  ============================
   (get i (combine'_l m1) = f (get' i m1) None)


DEBUG:Going to execute:
<coq-core.plugins.ltac::auto@0> $1 $2 $3