Goal:
    A : Type  l' : list A  v : nat  H : v < 0
  ============================
   (nth_error l' v = nth_error [] v)


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

In environment
A : Type
l' : list A
v : nat
H : v < 0
Unable to unify "nth_error [] v" with "nth_error l' v".
Level 0: Backtrace:

In environment
A : Type
l' : list A
v : nat
H : v < 0
Unable to unify "nth_error [] v" with "nth_error l' v".