;+
;
; @file_comments
; calculate umask. Economize memory (we do not need to keep umask anymore)
;
; @categories
; Grid
;
; @returns
; A 3d array corresponding to umask.
;
; @uses
; <pro>common</pro>
;
; @examples
; Used like if umask was a known array by substituting umask by (umask())
; For example: Instead of write umask[domainef] we have to write (umask())[domainef]
;
; @history
; Sebastien Masson (smasson\@lodyc.jussieu.fr)
;                       11/12/98
;                       ??/6/99 introduction of the key_shift
;                       20/9/99 cas jpk=1 (Thanks to  jpblod\@ipsl.jussieu.fr)
;
; @version
; $Id$
;
;-
FUNCTION umask
;
  compile_opt idl2, strictarrsubs
;
@cm_4mesh
  IF NOT keyword_set(key_forgetold) THEN BEGIN
@updatenew
  ENDIF
;---------------------------------------------------------
  tempsun = systime(1)          ; For key_performance
;
  CASE size(tmask, /n_dimensions) OF
    2:res = tmask*shift(tmask, -1, 0)
    3:res = tmask*shift(tmask, -1, 0, 0)
  ENDCASE
;
  if NOT keyword_set(key_periodic) then res[jpi-1, *, *] = umaskred
  if keyword_set(key_performance) THEN print, 'temps umask', systime(1)-tempsun
;
  return, res
end