`omit_fields` implies omitting `tiddler.fields` which is not what is actually happening. Clearly there's some naming mess going on here, but I reckon we need to continue to treat the word `fields` as "custom metadata on a tiddler". Perhaps `omit_attr` or `omit_attributes` as that is more aligned with the Python word for what is being omitted: a list of attributes of the tiddler which are to be omitted from the serialization.