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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 search(self, keyword, children=None, arg=None):
 """Return list of receiver's substmts with `keyword`. """

        if children is None:
            children = self.substmts
        return [ ch for ch in children
                 if (ch.keyword == keyword and
                     (arg is None or ch.arg == arg))]