Goal:
  
  A : Type
  B : Type
  map_InP : ∀ l : list A, (∀ x : A, In x l → B) → list B
  x0 : A
  xs : list A
  f : ∀ x : A, In x (x0 :: xs) → B
  x : A
  inx : In x xs
  ============================
   (x0 = x ∨ In x xs)


DEBUG:Going to execute:
match goal with
           | H:?X
             |- _ =>
                 match goal with
                 | H':?Y
                   |- _ =>
                       match H with
                       | H' => fail 2
                       | _ => unify X Y; clear H' || clear H
                       end
                 end
           end