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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_child(self, usage_id):
 """Return the child identified by ``usage_id``."""

        if usage_id in self._child_cache:
            return self._child_cache[usage_id]

        child_block = self.runtime.get_block(usage_id, for_parent=self)
        self._child_cache[usage_id] = child_block
        return child_block