public JComplex getComplex(Complex complex) {
   Enumeration e = elements();
   while (e.hasMoreElements()) {
     JComplex jcp = (JComplex) e.nextElement();
     Complex cp = jcp.getComplex();
     if (complex._is_equivalent(cp)) {
       // refer to the same object on server
       return jcp;
     }
   }
   // is better to throw an exception for this error
   return null;
 }