Goal:
  
  A : Type
  forallb_InP : ∀ l : list A, (∀ x : A, In x l → bool) → bool
  x : A
  xs : list A
  H : ∀ x0 : A, In x0 (x :: xs) → bool
  ============================
   (x = x ∨ In x xs)


DEBUG:Going to execute:
simpl proj1_sig in *
Level 5: Backtrace:

No matching clauses for match.
Level 5: Backtrace:

No matching clauses for match.
Level 4: Backtrace:

No matching clauses for match.
Level 4: Backtrace:

No matching clauses for match.
Level 3: Backtrace:

No matching clauses for match.
Level 3: Backtrace:

No matching clauses for match.
Level 2: Backtrace:

No matching clauses for match.
Level 2: Backtrace:

No matching clauses for match.
Level 1: Backtrace:

No matching clauses for match.
Level 0: Backtrace:

No matching clauses for match.
Level 0: Backtrace:

No matching clauses for match.