; return the filename for a single date and index.
; n = 1 if it is found, otherwise zero
; datestr yymmdd
; indx and n are integers
function fn1s,  datestr, indx, n

  if strlen(datestr) eq 6 then date6 = datestr else date6 = strmid(datestr, 2, 6)
  dir = '/Users/layoung/data/20201128_Eris_NIRES/*/'
  ; s190826_0049.fits
  spec = dir + 's'+date6+'_' + string(indx, for='(I04)') + '.fits'
  fn = file_search(spec, count=n)
  if n eq 0 then return, ''
  return, fn[0]

end