How can I figure out if a certain key exists in a mapping, Struct defined inside Library (in Solidity)?

pragma solidity ^0.4.8; library Library { struct data { unit val; } } contract Array{ using Library for Library.data; mapping(address => Library.data) clusterContract; function addCluster(address id) returns(bool){ if (clusterContract[id] == address(0)) { // error occurs! clusterContract[id] = list; return true; } return false; } } Following comparission if (clusterContract[id] == address(0)) give