A collection type that stores items of the same type. The item type must support toHash and ==
* Adds an item to the set. * Params: item to add.
* Removes all items leaving the set empty.
* Supports the in operator. * Params: item to check for membership in the set. * Returns: true if the item is in the set, or false if it isn't.
* Provides a range. * Returns: a range over the set's items.
* Attempts to remove the given item from the set. * Params: item to remove. * Returns: true if item present (and therefore removed) or false if * the item wasn't present.