function is triggered in the contract that is
    
    
    
    
    
    function approveAndCall(address _spender, uint256 _amount, bytes _extraData
    ) public returns (bool success) {
        require(approve(_spender, _amount));

        ApproveAndCallFallBack(_spender).receiveApproval(
            msg.sender,
            _amount,
            this,
            _extraData
        );

        return true;
    }