public String resolveSystemReverse(String systemId) throws IOException {
  Vector resolved=resolveAllSystemReverse(systemId);
  if (resolved != null && resolved.size() > 0) {
    return (String)resolved.elementAt(0);
  }
 else {
    return null;
  }
}

public String resolveSystemReverse ( String systemId ) throws IOException { Vector resolved = resolveAllSystemReverse ( systemId ) ; if ( resolved != null && resolved . size ( ) > 0 ) { return ( String ) resolved . elementAt ( 0 ) ; } else { return null ; } }
find the urn for a given system identifier .
