Goal:
    A : Type  d : A  n : nat  v : A  H : d = v
  ============================
   (v = d)


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

In environment
A : Type
d : A
n : nat
v : A
H : d = v
Unable to unify "d" with "v".
Level 0: Backtrace:

In environment
A : Type
d : A
n : nat
v : A
H : d = v
Unable to unify "d" with "v".