function transferFrom(address _from, address _to, uint256 _value) blockLock(_from) checkIfToContract(_to) returns (bool success) {
    if (balances[_from] >= _value && allowed[_from][msg.sender] >= _value && _value > 0) {
      balances[_to] += _value;
      balances[_from] -= _value;
      allowed[_from][msg.sender] -= _value;
      Transfer(_from, _to, _value);
      return true;
    } else {
      return false;
    }
  }