function burnFrom(address _from, uint256 _value) public returns (bool success) {
            require(balances[_from] >= _value);                                      
            require(_value <= allowed[_from][msg.sender]);                          
            balances[_from] =  balances[_from].sub(_value);                         
            allowed[_from][msg.sender] =   allowed[_from][msg.sender].sub(_value);   
            totalSupply = totalSupply.sub(_value);                                      
            emit Burn(_from, _value);
            return true;
        }