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Task/Set/00DESCRIPTION

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+{{data structure}}
+A set is a collection of elements, without duplicates and without order.
+
+Show each of these set operations:
+
+* Set creation
+* Test m ∈ S -- "m is an element in set S"
+* A ∪ B -- ''union''; a set of all elements either in set A or in set B.
+* A ∩ B -- ''intersection''; a set of all elements in ''both'' set A and set B.
+* A ∖ B -- ''difference''; a set of all elements in set A, except those in set B.
+* A ⊆ B -- ''subset''; true if every element in set A is also in set B.
+* A = B -- ''equality''; true if every element of set A is in set B and vice-versa.
+
+As an option, show some other set operations. (If A ⊆ B, but A ≠ B, then A is called a true or proper subset of B, written A ⊂ B or A ⊊ B.) As another option, show how to modify a mutable set.
+
+One might implement a set using an [[associative array]] (with set elements as array keys and some dummy value as the values). One might also implement a set with a binary search tree, or with a hash table, or with an ordered array of binary bits (operated on with bitwise binary operators). The basic test, m ∈ S, is [[O]](n) with a sequential list of elements, O(''log'' n) with a balanced binary search tree, or (O(1) average-case, O(n) worst case) with a hash table.