;+
; NAME:
;	SMEI_NULL_INDEX
;
;
; PURPOSE:
;	To set the colour index/indices to use for missing data.
;
;
; CATEGORY:
;	CLI
;
;
; CALLING SEQUENCE:
;	smei_null_index, seqref, index[, /screen|/print]
;
;
; INPUTS:
;	seqref	objref	Object reference to the sequence.
;	index	byte	The colour index to use (or a 2-element list).
;
;
; KEYWORD PARAMETERS:
;	/screen		If set, then only set the index for screen displays.
;	/print		If set, then only set the index for printouts.
;	/off_sky	If set, then set the value to use for
;			points outside the sky area
;
;
; MODIFICATION HISTORY:
;	Original: 11/7/03; SJT
;	Separate in and out of sky: 18/7/03; SJT
;-

pro smei_null_index, seqref, index, print = print, screen = screen, $
                     off_sky = off_sky, help = help

if keyword_set(help) then begin
    self_help
    return
end

if not obj_valid(seqref) then begin
    smei_msg, /alert,  'SMEI_NULL_INDEX requires an object reference'
    return
endif

if not obj_isa(seqref, 'SMEI_SEQUENCE') then begin
    smei_msg, /alert, $
              'The object you are trying to modify is not a ' + $
              'SMEI_SEQUENCE'
    return
endif

seqref -> set_null_index, index, screen = screen, print = print, $
  off_sky = off_sky

end