CombinationLock implementation
// entered1, entered2, entered3 are the last
three
// digits entered, with entered3 the most
recent.
// a value of -1 indicates the digit has not
been
// entered.
private int entered1;
private int entered2;
private int entered3;
// invariant:
// entered1 >= -1 &&
entered1 <= 9 &&
// entered2 >= -1 &&
entered2 <= 9 &&
// entered3 >= -1 &&
entered3 <= 9