function burnFrom(address _from, uint256 _value) public returns (bool success) {
        require(block.timestamp > lockEndTime);
        require(balanceOf[_from] >= _value);                
        require(_value <= allowance[_from][msg.sender]);    
        balanceOf[_from] -= _value;                         
        allowance[_from][msg.sender] -= _value;             
        totalSupply -= _value;                              
        emit Burn(_from, _value);
        return true;
    }