Goal:
  
  A : Type
  B : Type
  f : A → B
  a : A
  l : list A
  H : a :: l ≠ []
  e : map f (a :: l) = []
  ============================
   (a :: l = [])


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

In environment
A : Type
B : Type
f : A → B
a : A
l : list A
H : a :: l ≠ []
e : map f (a :: l) = []
Unable to unify "[]" with "a :: l".
Level 0: Backtrace:

In environment
A : Type
B : Type
f : A → B
a : A
l : list A
H : a :: l ≠ []
e : map f (a :: l) = []
Unable to unify "[]" with "a :: l".