function _transfer(address _from, address _to, uint256 _value) internal
    {
      require(_to != 0x0);
 
      require(balanceOf[_from] >= _value);
 
      require(balanceOf[_to] + _value > balanceOf[_to]);

      uint previousBalances = balanceOf[_from] + balanceOf[_to];
 
      balanceOf[_from] -= _value;

      balanceOf[_to] += _value;

      Transfer(_from, _to, _value);

      assert(balanceOf[_from] + balanceOf[_to] == previousBalances);
    }