Goal:
    z : t  hd : A.t  tl : list A.t  H1' : list A.t  H1 : t  H : lt_ H1' H1
  ============================
   ({| pr1 := hd :: tl; pr2 := z |} = {| pr1 := H1'; pr2 := H1 |} ->
    let H := CoreTactics.block in let H0 := CoreTactics.block in lt [] 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 := H1
of type
constr of type tacvalue