pro nsxng_set_root, IA, IB, IAB

; Roots
;> the part of the filename basename without the ',fits'
;> We can use the IDL routine "file_basename"
  IA.root = file_basename(IA.file, '.fits')

  if n_params() ge 2 then begin
     if (IB.X) then begin
        IB.root = file_basename(IB.file, '.fits')
;        print, IA.file,IB.file, for='("Found 2 image files: ",A," and ",A,".")'
     endif else begin
;        print, IA.file,for='("Found 1 image file: ",A,".")'
     endelse
  endif

  if n_params() ge 3 then begin
; IAB image will be copied from IA, and new filename includes unique part of rootB.
     if (IB.X) then begin
        IAB.file = IA.file
        kk = 0
        while(strmid(IA.root,0,kk) eq strmid(IB.root,0,kk)) do kk++
        IAB.root = IA.root + '-' + strmid(IB.root, kk-1, strlen(IB.root) )
     endif
  endif
  
end