code: def compute_texptime(imageObjectList):
      
      expnames = []
      exptimes = []
      start = []
      end = []
      for img in imageObjectList:
          expnames += img.getKeywordList('_expname')
          exptimes += img.getKeywordList('_exptime')
          start += img.getKeywordList('_expstart')
          end += img.getKeywordList('_expend')
  
      exptime = 0.
      expstart = min(start)
      expend = max(end)
      exposure = None
      for n in range(len(expnames)):
          if expnames[n] != exposure:
              exposure = expnames[n]
              exptime += exptimes[n]
  
      return (exptime,expstart,expend)
docs: Add up the exposure time for all the members in
      the pattern, since 'drizzle' doesn't have the necessary
      information to correctly set this itself.