Goal:
    A : Type  B : Type  C : Type  P : A → B → C → Type  b : B  ls3 : list B
  ============================
   ([] = [])


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.