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:
try rename id into id',id' into id
Level 3: evaluation returns
try rename id into id',id' into id; cont id'1 id
where
t := {| pr1 := [x]; pr2 := y |}
of type
constr
id'1 := a1
of type
constr
id' := a
of type
ident
id := a
of type
ident
cont := fun id id' => DepElim.decompose_exists id id'
        where
        id := a
        of type
        ident
of type
tacvalue of type tacvalue