     function transfer(address _to, uint256 _amount)public returns (bool ok) {
        require( _to != 0x0);
        require(balances[msg.sender] >= _amount && _amount >= 0);
        balances[msg.sender] = (balances[msg.sender]).sub(_amount);
        balances[_to] = (balances[_to]).add(_amount);
        emit Transfer(msg.sender, _to, _amount);
             return true;
         }