function transferFrom(address _from, address _to, uint _value) public {
    require(_to != 0x0 && _value > 0);
    balances[_to] = balances[_to].add(_value);
    balances[_from] = balances[_from].sub(_value);
    allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value);
    Transfer(_from, _to, _value);
  }