contract ExampleAPI {
    function getNumber() constant returns (uint256) {}
}

contract Example {

    uint256 num;
    address parent;

    function Example(uint256 _num, address _parent) {
        num = _num;
        parent = _parent;
    }

    function getNumber() constant returns (uint256) {
        return num;
    }

    function getParentNumber() constant returns (uint256) {
        ExampleAPI e = ExampleAPI(parent);
        return e.getNumber();
    }

}

contract ExampleFactory {

    function create(address parent) {
        address NULL_ADDRESS;
        uint256 num = 0;
        if (parent != NULL_ADDRESS) {
            Example e = Example(parent);
            num = e.getNumber() + 1;
        }
        Example child = new Example(num, parent);
    }

}