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Another thing that lists have been used for is set manipulation. In
applications where there is a known, reasonably small universe of
items bit-vectors can be used to improve performance. This is much
less convenient than using lists, because instead of symbols, each
element in the universe must be assigned a numeric index into the bit
vector. Using a bit-vector will nearly always be faster, and can be
tremendously faster if the number of elements in the set is not small.
The logical operations on simple-bit-vectors are efficient,
since they operate on a word at a time.