func: 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);
      }