Goal:
    z : Z  z0 : Z  z1 : Z  z2 : Z  H : (z <= z0)%Z  H0 : (z1 <= z2)%Z
  ============================
   (z + z1 <= z0 + z2)%Z


DEBUG:Going to execute:
<Coq.Init.Tauto.is_empty> flags X1; elim H
Level 3: evaluation returns
<coq-core.plugins.tauto::is_unit_or_eq@0>
where
tauto_flags := 
of type
tauto_flags
X1 := (z + z1 <= z0 + z2)%Z
of type
constr of type tacvalue

Level 3: Backtrace:

Tactic failure.
Level 3: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.