code: public static String prefixFromPlain(String u) {
          u = fromPlain(u);
          // truncate to implied prefix
          u = SurtPrefixSet.asPrefix(u);
          return u;
      }
docs: Given a plain URI or hostname/hostname+path, deduce an implied SURT
  prefix from it. Results may be unpredictable on strings that cannot
  be interpreted as URIs.
  
  UURI 'fixup' is applied to the URI that is built.
  
  @param u URI or almost-URI to consider
  @return implied SURT prefix form