Goal:
  
  A : Type
  i : positive
  x : A
  IHi : forall m : tree' A, get' i (set' i x m) = Some x
  m1 : tree' A
  m2 : tree' A
  ============================
   (get' i (set' i x m2) = Some x)


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