docs: def get_cands_uri(field, ccd, version='p', ext='measure3.cands.astrom', prefix=None,
                    block=None):
      """
          return the nominal URI for a candidate file.
  
      @param field: the OSSOS field name
      @param ccd: which CCD are the candidates on
      @param version: either the 'p', or 's' (scrambled) candidates.
      @param ext: Perhaps we'll change this one day.
      @param prefix: if this is a 'fake' dataset then add 'fk'
      @param block: Which BLOCK of the field are we looking at? eg. 15BS+1+1
      @return:
      """
  
      if prefix is None:
          prefix = ""
      if len(prefix) > 0:
          prefix += "_"
      if len(field) > 0:
          field += "_"
  
      if ext is None:
          ext = ""
      if len(ext) > 0 and ext[0] != ".":
          ext = ".{}".format(ext)
  
      measure3_dir = MEASURE3
      if block is not None:
          measure3_dir + "/{}".format(block)
      return "{}/{}{}{}{}{}".format(measure3_dir, prefix, field, version, ccd, ext)