function transferFrom(address _from, address _to, uint256 _amount) public returns (bool) {
    require(_to != address(0));
    require(_amount <= balances[_from]);
    require(_amount <= allowed[_from][msg.sender]);
    require(block.timestamp > lockups[_from]);
    require(block.timestamp > lockups[_to]);

    balances[_from] = balances[_from].sub(_amount);
    balances[_to] = balances[_to].add(_amount);
    allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_amount);
    emit Transfer(_from, _to, _amount);
    return true;
  }