Returns true if the two sets have the same membership, false if not
capacity
Answer how many elements the receiver can hold before having to grow.
hash
Return the hash code for the members of the set. Since order is
unimportant, we use a commutative operator to compute the hash value.
includes: anObject
Answer whether the receiver contains an instance of anObject.
isEmpty
Answer whether the receiver is empty.
occurrencesOf: anObject
Return the number of occurrences of anObject. Since we're a set, this
is either 0 or 1. Nil is never directly in the set, so we special case
it (the result is always 1).