code: def _load_prefix_binding(self):
          
          pymux = self.pymux
  
          # Remove previous binding.
          if self._prefix_binding:
              self.custom_key_bindings.remove_binding(self._prefix_binding)
  
          # Create new Python binding.
          @self.custom_key_bindings.add(*self._prefix, filter=
              ~(HasPrefix(pymux) | has_focus(COMMAND) | has_focus(PROMPT) |
                WaitsForConfirmation(pymux)))
          def enter_prefix_handler(event):
              " Enter prefix mode. "
              pymux.get_client_state().has_prefix = True
  
          self._prefix_binding = enter_prefix_handler
docs: Load the prefix key binding.