in switch.py [0:0]
def default(self, p: Processor | PartProcessor) -> Self:
if self._default_set:
raise ValueError('The default processor is already set.')
if isinstance(p, PartProcessor):
self._cases.append((lambda x: True, p.to_processor()))
else:
self._cases.append((lambda x: True, p))
self._default_set = True
return self