Goal:
  
  A : Type
  x : A
  m1 : tree A
  o : option A
  m2 : tree A
  H : not_trivially_empty m1 o m2
  IHm1 : xH <> xH -> get xH (set xH x m1) = get xH m1
  IHm2 : xH <> xH -> get xH (set xH x m2) = get xH m2
  H0 : xH <> xH
  ============================
   (get xH (set xH x (Node m1 o m2)) = get xH (Node m1 o m2))


DEBUG:Going to execute:
rewrite set_Empty
DEBUG LTAC Evaluated term: set_Empty
DEBUG LTAC Evaluated term: set_Empty