Goal:
    hd' : A.t  tl' : list A.t  hd : A.t  tl : list A.t  H : A.lt hd hd'
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in lt [] (hd' :: tl'))


DEBUG:Going to execute:
change T; CoreTactics.intros_until_block
Level 3: T
 has value 
(let H := CoreTactics.block in lt [] (hd' :: tl'))