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6.21.5 Extending `optparse`

Since the two major controlling factors in how `optparse` interprets command-line options are the action and type of each option, the most likely direction of extension is to add new actions and new types.

Also, the `examples/` directory of the source distribution includes several demonstrations of extending `optparse` in different ways: e.g. a case-insensitive option parser, or two kinds of option parsers that implement ``required options''.