Goal:
  
  hd' : A.t
  tl'0 : t
  hd : A.t
  tl : list A.t
  tl' : list A.t
  H : A.lt hd hd'
  H' : lt_ tl' tl'0
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in lt (hd :: tl) (hd' :: tl'0))


DEBUG:Going to execute:
change T; CoreTactics.intros_until_block
Level 3: T
 has value 
(let H := CoreTactics.block in lt (hd :: tl) (hd' :: tl'0))