Goal:
    z : Z  k : t  H : Some z = k
  ============================
   (None = k \/ Some z = k)


DEBUG:Going to execute:
match goal with
           | |- ?X1 -> ?X2 => intro
           | |- ~ _ => unfold not at 1; intro
           end
Level 0: Backtrace:

No matching clauses for match.