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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 register_callback(self, key):
 """Adds ``key`` to the list of callbacks."""

        if self.pending_callbacks is None:
            # Lazily initialize the old-style YieldPoint data structures.
            self.pending_callbacks = set()
            self.results = {}
        if key in self.pending_callbacks:
            raise KeyReuseError("key %r is already pending" % (key,))
        self.pending_callbacks.add(key)