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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_commands(self, source=None):
 """Return a string containing multiple `reStructuredText` replacements with the substitutions currently defined. Some examples based on the subpackage |optiontools|: When calling |Substituter.get_commands| with the `source` argument, the complete `short2long` and `medium2long` mappings are translated into replacement commands (only a few of them are shown):
 .. |Options.autocompile| replace:: \ :const:`~hydpy.core.optiontools.Options.autocompile` .. |Options.checkseries| replace:: \ :const:`~hydpy.core.optiontools.Options.checkseries` .. |optiontools.Options.warntrim| replace:: \ :const:`~hydpy.core.optiontools.Options.warntrim` .. |optiontools.Options| replace:: \ :class:`~hydpy.core.optiontools.Options` Through passing a string (usually the source code of a file to be documented), only the replacement commands relevant for this string are translated: .. |Options.reprdigits| replace:: \ :const:`~hydpy.core.optiontools.Options.reprdigits` """

        commands = []
        for key, value in self:
            if (source is None) or (key in source):
                commands.append('.. %s replace:: %s' % (key, value))
        return '\n'.join(commands)