anchor.diff:  s_null
   :
     (
  -    BOOT
  -    | CLI
  +    CLI
       | CLOCK
       | ERRDISABLE
       | FEATURE
  
positive.code:   BOOT
    (
      boot_kickstart
      | boot_nxos
      | boot_system
    )
  
negative.code: boot_kickstart
  :
    KICKSTART image = WORD (SUP_1 | SUP_2)? NEWLINE
  ;
  
  boot_null
  :
    (
      POAP
    ) null_rest_of_line
  ;
  
  boot_nxos
  