code: def _add_call_site(self, call_site_addr, call_target_addr, retn_addr):
          
          self._call_sites[call_site_addr] = (call_target_addr, retn_addr)
docs: 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.