Briggs-Torczon sets of small integers (0 <= x < N) deserve to be better known.
(Set up two N-element arrays "member" and "index" of ints and an elementCount; set elementCount to zero. You don't have to initialize the arrays if you can live with some valgrind grumbling. The leading "elementCount" entries in "member" are the current members of the set, unordered, and for each of those values x, index[x] is its index in "members".
Then isInSet(x) = index[x] >= 0 && index[x] < elementCount && member[index[x]] == x
and addToSet(x) = isInSet(x) || (member[index[x] = elementCount++] = x)
(Set up two N-element arrays "member" and "index" of ints and an elementCount; set elementCount to zero. You don't have to initialize the arrays if you can live with some valgrind grumbling. The leading "elementCount" entries in "member" are the current members of the set, unordered, and for each of those values x, index[x] is its index in "members".
Then isInSet(x) = index[x] >= 0 && index[x] < elementCount && member[index[x]] == x and addToSet(x) = isInSet(x) || (member[index[x] = elementCount++] = x)
and rmFromSet() is left as an exercise.)