Goal:
  
  A : Type
  P : A → A → Type
  a : A
  l : list A
  l' : list A
  f : A → A
  hp : ∀ x y : A, P (f x) y → ∑ y' : A, y = f y'
  IHl : ∀ l' : list A,
          OnOne2 P (map f l) l'
          → ∑ l'' : list A, OnOne2 (λ x y : A, P (f x) (f y)) l l''
  o : OnOne2 P (f a :: map f l) l'
  o0 := {| pr1 := {| pr1 := f a :: map f l; pr2 := l' |}; pr2 := o |}
     : sigma
         (λ index : sigma (λ _ : list A, list A),
            OnOne2 P (pr1 index) (pr2 index))
  o1 := {| pr1 := f a :: map f l; pr2 := l' |} : sigma (λ _ : list A, list A)
  o2 := o
     : (λ index : sigma (λ _ : list A, list A),
          OnOne2 P (pr1 index) (pr2 index)) o1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    ∑ l'' : list A, OnOne2 (λ x y : A, P (f x) (f y)) (a :: l) l'')


DEBUG:Going to execute:
match goal with
           | id'1:=?t:_
             |- _ =>
                 generalize (eq_refl : t = id'1); clearbody id'2 id'1; clear
                  id' id; try unfold signature in id'2; 
                  hnf in id'2; simpl in id'2; rename id'2 into id;
                  cont id id id'1 t
           end
Level 1: Backtrace:

No head constant to reduce.