code: def verifyRefimage(refimage):
      
      valid = True
  
      # start by trying to see whether the code can even find the file
      if is_blank(refimage):
          valid=True
          return valid
  
      refroot,extroot = fileutil.parseFilename(refimage)
      if not os.path.exists(refroot):
          valid = False
          return valid
  
      # if a MEF has been specified, make sure extension contains a valid WCS
      if valid:
          if extroot is None:
              extn = findWCSExtn(refimage)
              if extn is None:
                  valid = False
              else:
                  valid = True
          else:
              # check for CD matrix in WCS object
              refwcs = wcsutil.HSTWCS(refimage)
              if not refwcs.wcs.has_cd():
                  valid = False
              else:
                  valid = True
              del refwcs
  
      return valid
docs: Verify that the value of refimage specified by the user points to an
      extension with a proper WCS defined. It starts by making sure an extension gets
      specified by the user when using a MEF file. The final check comes by looking
      for a CD matrix in the WCS object itself. If either test fails, it returns
      a value of False.