Goal:
  
  A : Type
  B : Type
  P : list A → list A → A → A → B → Type
  ls2 : list A
  ls3 : list A
  H : [] ≠ []
  ============================
   (#|ls2| = #|ls3|)


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

In environment
A : Type
B : Type
P : list A → list A → A → A → B → Type
ls2, ls3 : list A
H : [] ≠ []
Unable to unify "#|ls3|" with "#|ls2|".
Level 0: Backtrace:

In environment
A : Type
B : Type
P : list A → list A → A → A → B → Type
ls2, ls3 : list A
H : [] ≠ []
Unable to unify "#|ls3|" with "#|ls2|".