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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_one(self, keyword, arg=None, children=None):
 """Return receiver's substmt with `keyword` and optionally `arg`. """

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