// Disconnect
/** Disconnect me from my current successor, if any. */
        void disconnect() {
            Sq next = _successor;
            if (next == null) {
                return;
            }
            _unconnected += 1; // increase number of squares that haven't been connected
            next._predecessor = _successor = null;
            if (_sequenceNum == 0) {
                // FIXME: If both this and next are now one-element groups,
                //        release their former group and set both group
                //        numbers to -1.
                //        Otherwise, if either is now a one-element group, set
                //        its group number to -1 without releasing the group
                //        number.
                //        Otherwise, the group has been split into two multi-
                //        element groups.  Create a new group for next.
                if ((this._predecessor == null) && (next._successor == null)) { // Cond 1
                    releaseGroup(this._head._group);
                    this._head._group = -1;
                    next._head._group = -1;
                }
                else if ((this._predecessor == null) || (next._successor == null)) { // Cond 2
                    if ((this._predecessor == null))
                        this._head._group = -1;
                    else
                        next._head._group = -1;
                }
                else {
                    next._head._group = newGroup();
                }
            } else {
                // FIXME: If neither this nor any square in its group that
                //        precedes it has a fixed sequence number, set all
                //        their sequence numbers to 0 and create a new group
                //        for them if this has a current predecessor (other
                //        set group to -1).
                // FIXME: If neither next nor any square in its group that
                //        follows it has a fixed sequence number, set all
                //        their sequence numbers to 0 and create a new
                //        group for them if next has a current successor
                //        (otherwise set next's group to -1.)
                boolean noFixedNums = this._hasFixedNum;
                Sq predecessorPtr = this._predecessor;
                while (predecessorPtr != null) {
                    boolean tempBool = false;
                    if (!(predecessorPtr._hasFixedNum))
                        tempBool = true;
                    noFixedNums = (noFixedNums && tempBool);
                    predecessorPtr = predecessorPtr._predecessor;
                }
                if (noFixedNums) { // condition #1
                    this._sequenceNum = 0;
                    Sq predecessorPtr2 = this._predecessor;
                    while (predecessorPtr2 != null) {
                        predecessorPtr2._sequenceNum = 0;
                        predecessorPtr2 = predecessorPtr2._predecessor;
                    }
                    if (this._predecessor != null) {
                        this._head._group = newGroup();
                    }
                    else {
                        this._head._group = -1;
                    }
                }
                boolean noFixedNums2 = next._hasFixedNum;
                Sq successorPtr = next._successor;
                while (successorPtr != null) {
                    boolean tempBool = false;
                    if (!(successorPtr._hasFixedNum))
                        tempBool = true;
                    noFixedNums2 = (noFixedNums2 && tempBool);
                    successorPtr = successorPtr._successor;
                }
                if (noFixedNums2) { // condition #2
                    next._sequenceNum = 0;
                    Sq successorPtr2 = next._successor;
                    while (successorPtr2 != null) {
                        successorPtr2._sequenceNum = 0;
                        successorPtr2 = successorPtr2._successor;
                    }
                    if (next._successor != null) {
                        next._head._group = newGroup();
                    }
                    else {
                        next._head._group = -1;
                    }
                }
            }
            // FIXME: Set the _head of next and all squares in its group to
            //        next.
            next._head = next;
            Sq successorPtr = next._successor;
            while (successorPtr != null) {
                successorPtr._head = next;
                successorPtr = successorPtr._successor;
            }
        }