Goal:
  
  z : t
  hd : A.t
  tl : list A.t
  H1 : sigma (fun _ : t => t)
  H : lt_ (pr1 H1) (pr2 H1)
  ============================
   ({| pr1 := hd :: tl; pr2 := z |} = H1 ->
    let H := CoreTactics.block in let H0 := CoreTactics.block in lt [] z)


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 := hd :: tl; pr2 := z |}
of type
constr
id'1 := H1
of type
constr
id' := H
of type
ident
id := H
of type
ident
cont := fun id id' => DepElim.decompose_exists id id'
        where
        id := H
        of type
        ident
of type
tacvalue of type tacvalue