Today's Little Program prints all bit strings of length n
subject to the requirement that the string have exactly k
1-bits.
For example, all bit strings of length 4 with 2 bits set are
0011, 0101, 0110, 1001, 1010, and 1100.
Let's write
BitString(n, k) to mean all the bit strings
of length n with exactly k bits set.
Let's look at the ...