Goal:
    x : string  y : string  z : string  H : lt x y  H0 : lt z y
  ============================
   (Gt = Lt -> compare x z = Lt)


DEBUG:Going to execute:
congruence
Level 0: Backtrace:

Tactic failure: congruence failed.
Level 0: Backtrace:

Tactic failure: congruence failed.