Goal:
  
  A : Type
  B : Type
  R : A → B → Prop
  x : A
  y : list B
  a : Forall2 R [x] y
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in ∃ y' : B, y = [y'] ∧ R x y')


DEBUG:Going to execute:
fun id =>
             DepElim.generalize_sig id
              ltac:(fun id id' => DepElim.decompose_exists id id')