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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 _add_call_site(self, call_site_addr, call_target_addr, retn_addr):
 """ Registers a basic block as calling a function and returning somewhere. :param call_site_addr: The address of a basic block that ends in a call. :param call_target_addr: The address of the target of said call. :param retn_addr: The address that said call will return to. """

        self._call_sites[call_site_addr] = (call_target_addr, retn_addr)