code: public X509Extension add(X509Extension extension) {
  	if (extension == null) {
  	    throw new IllegalArgumentException(i18n
                                                 .getMessage("extensionNull"));
  	}
  	return (X509Extension)this.extensions.put(extension.getOid(),
  						  extension);
      }
docs: Adds a X509Extension object to this set.
  
  @param extension the extension to add
  @return an extension that was removed with the same oid as the
  new extension. Null, if none existed before.