##
5.15.2 Example

    >>> from sets import Set
    >>> engineers = Set(['John', 'Jane', 'Jack', 'Janice'])
    >>> programmers = Set(['Jack', 'Sam', 'Susan', 'Janice'])
    >>> managers = Set(['Jane', 'Jack', 'Susan', 'Zack'])
    >>> employees = engineers | programmers | managers           # union
    >>> engineering_management = engineers & managers            # intersection
    >>> fulltime_management = managers - engineers - programmers # difference
    >>> engineers.add('Marvin')                                  # add element
    >>> print engineers
    Set(['Jane', 'Marvin', 'Janice', 'John', 'Jack'])
    >>> employees.issuperset(engineers)           # superset test
    False
    >>> employees.union_update(engineers)         # update from another set
    >>> employees.issuperset(engineers)
    True
    >>> for group in [engineers, programmers, managers, employees]:
    ...     group.discard('Susan')                # unconditionally remove element
    ...     print group
    ...
    Set(['Jane', 'Marvin', 'Janice', 'John', 'Jack'])
    Set(['Janice', 'Jack', 'Sam'])
    Set(['Jane', 'Zack', 'Jack'])
    Set(['Jack', 'Sam', 'Jane', 'Marvin', 'Janice', 'John', 'Zack'])