docs: def get_default(self):
          """
          If the user provided a default in the field definition, returns it,
          otherwise determines the default menus based on available choices and
          ``PAGE_MENU_TEMPLATES_DEFAULT``. Ensures the default is not mutable.
          """
          if self._overridden_default:
              # Even with user-provided default we'd rather not have it
              # forced to text. Compare with Field.get_default().
              if callable(self.default):
                  default = self.default()
              else:
                  default = self.default
          else:
              # Depending on PAGE_MENU_TEMPLATES_DEFAULT:
              # * None or no value: all choosable menus;
              # * some sequence: specified menus;
              # (* empty sequence: no menus).
              default = getattr(settings, "PAGE_MENU_TEMPLATES_DEFAULT", None)
              if default is None:
                  choices = self.get_choices(include_blank=False)
                  default = (c[0] for c in choices)
          # Default can't be mutable, as references to it are shared among
          # model instances; all sane values should be castable to a tuple.
          return tuple(default)