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


DEBUG:Going to execute:
symmetry; trivial
Level 1: Backtrace:

In environment
z, z0 : Z
H : (z <= z0)%Z
H0 : (z0 <= z)%Z
Unable to unify "Some z0" with "Some z".
Level 0: Backtrace:

In environment
z, z0 : Z
H : (z <= z0)%Z
H0 : (z0 <= z)%Z
Unable to unify "Some z0" with "Some z".