def fetch(code, year = nil, opts = {})
      stdclass = standard_class(code) or return nil
      check_bibliocache(code, year, opts, stdclass)
    end
def fetch(code, year = nil, opts = {})
      stdclass = standard_class(code) or return nil
      check_bibliocache(code, year, opts, stdclass)
    end
The class of reference requested is determined by the prefix of the code:
 GB Standard for gbbib, IETF for ietfbib, ISO for isobib, IEC or IEV for iecbib,
 @param code [String] the ISO standard Code to look up (e.g. "ISO 9000")
 @param year [String] the year the standard was published (optional)
 @param opts [Hash] options; restricted to :all_parts if all-parts reference is required
 @return [String] Relaton XML serialisation of reference