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