KeyNode node = (KeyNode)keys.elementAt( pos );
pos = node.start_position;
element_replace( pos, element );
return true;
} catch ( ObjectInvalid e ){
throw new ElementInvalid( ElementInvalidReason.element_type_invalid );
} catch ( PositionInvalid e ){
throw new ElementInvalid( ElementInvalidReason.positioning_property_invalid );
}
};