Goal:
  
  A : Type
  B : Type
  P : list A → list A → A → A → B → Type
  a : A
  ls2 : list A
  ============================
   ([] = [])


DEBUG:Going to execute:
exactly_once constructor
Level 1: Backtrace:

No matching clauses for match.
Level 1: Backtrace:

No matching clauses for match.
Level 0: Backtrace:

No matching clauses for match.