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


DEBUG:Going to execute:
match goal with
           | |- let _ := CoreTactics.block in _ => intros _
           | |- _ => try (intro; CoreTactics.intros_until_block)
           end