Goal:
  
  A : Type
  B : Type
  R : A → B → Prop
  x : A
  y0 : B
  l' : list B
  H : R x y0
  a : Forall2 R [] l'
  a0 := {| pr1 := {| pr1 := []; pr2 := l' |}; pr2 := a |}
     : sigma
         (λ index : sigma (λ _ : list A, list B),
            Forall2 R (pr1 index) (pr2 index))
  a1 := {| pr1 := []; pr2 := l' |} : sigma (λ _ : list A, list B)
  a2 := a
     : (λ index : sigma (λ _ : list A, list B),
          Forall2 R (pr1 index) (pr2 index)) a1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in ∃ y' : B, y0 :: l' = [y'] ∧ R x y')


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.