Goal:
    z : Z  z0 : Z
  ============================
   ((z <= z0)%Z -> (z0 <= z)%Z -> Some z = Some z0)


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

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.
Level 1: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.