function MultiSigWallet(address[] _owners, uint _required)
        public
        validRequirement(_owners.length, _required)
    {
        for (uint i=0; i<_owners.length; i++) {
            if (isOwner[_owners[i]] || _owners[i] == 0)
                revert();
            isOwner[_owners[i]] = true;
        }
        owners = _owners;
        required = _required;
    }