Goal:
  
  A : Type
  m1 : tree' A
  a : A
  m2 : tree' A
  IHm1 : exists i : positive, get' i m1 <> None
  IHm2 : exists i : positive, get' i m2 <> None
  ============================
   (exists i : positive, get' i (Node111 m1 a m2) <> None)


DEBUG:Going to execute:
exists xH