function _transfer(
        address _to, 
        uint256 _value
    ) 
    internal
    nonReentrant
    returns (bool) 
    {
        require(_to != address(0));
        require(_value <= balances[msg.sender]);

        balances[msg.sender] = balances[msg.sender].sub(_value);
        balances[_to] = balances[_to].add(_value);
        emit Transfer(msg.sender, _to, _value);
        emit Transfer(msg.sender, msg.sender, _to, _value);
        return true;
    }