Goal:
    A : Type  x : A  m1 : tree' A  m2 : tree' A  H : xH <> xH
  ============================
   (Some x = None)


DEBUG:Going to execute:
apply gso0; congruence
Level 0: Backtrace:

The variable IHi was not found in the current environment.
Level 0: Backtrace:

The variable IHi was not found in the current environment.