[From the notes of Herb Wilf, transcribed by Emily Moore. Modified and extended by Sam Rebelsky.]

It is possible to apply Gray code techniques to "strings" of characters
as well as sets. Consider the set of strings of length three in which
all the characters in the string are 0 or 1. These are typically
called *binary strings*. (Binary means "two-valued".)

B3: { 000, 001, 010, 011, 100, 101, 110, 111 }

The order in which they are listed here is called their natural order. If you think of these as binary numbers with place values from right to left of 1, 2, and 4, then the order is the order of their values 0, 1, 2, 3, 4, 5, 6, 7. For example, 111 is 4 + 2 + 1 = 7 and 010 is 0 + 2 + 0 = 2.

A *Gray code order* of these strings is a listing of the strings
so that two strings next to each other differ only in one position.
For instance, if we start with 000 the next string listed must have a
single 1.

A. Find a listing of all eight strings in Gray code order.

B. Is there more than one such listing? If so, how many listings are there?

C. Find a listing of all sixteen length-four binary strings in Gray code order.

D. Can you generalize the pattern?

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