Goal:
    hd : A.t  tl' : t  tl : list A.t  H : lt_ tl tl'
  ============================
   (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'))