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


DEBUG:Going to execute:
destruct id as [xn id]; DepElim.decompose_exists xn id;
            cbv beta iota delta [TODO projection TODO projection] in id, id';
            DepElim.decompose_exists id id'