Goal:
  
  z : t
  hd : A.t
  tl : list A.t
  hd' : A.t
  tl' : list A.t
  H : A.lt hd hd'
  H'1 : sigma (fun _ : t => t)
  H' : lt_ (pr1 H'1) (pr2 H'1)
  ============================
   ({| pr1 := hd' :: tl'; pr2 := z |} = H'1 ->
    let H := CoreTactics.block in
    let H0 := CoreTactics.block in lt (hd :: tl) z)


DEBUG:Going to execute:
hnf in id
Level 4: evaluation returns
hnf in id;
 match type of id with
 | sigma _ =>
     let xn := fresh id "'" in
     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'
 | _ => cbv beta iota delta [TODO projection TODO projection] in id, id'
 end
where
id' := H'
of type
ident
id := H'1
of type
constr of type tacvalue

Level 3: evaluation returns
hnf in id;
 match type of id with
 | sigma _ =>
     let xn := fresh id "'" in
     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'
 | _ => cbv beta iota delta [TODO projection TODO projection] in id, id'
 end
where
id' := H'
of type
ident
id := H'1
of type
constr of type tacvalue