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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 user(self, user: str) -> "ChildHTTPAPI": """ Get a child HTTPAPI instance. Args: user: The Matrix ID of the user whose API to get. Returns: A HTTPAPI instance that always uses the given Matrix ID. """

        if self.is_real_user:
            raise ValueError("Can't get child of real user")

        try:
            return self.children[user]
        except KeyError:
            child = ChildHTTPAPI(user, self)
            self.children[user] = child
            return child