[wip] 8413 and 8373 related changes

See #8413 and #8373 

- [ ] check coverage, especially the handling of duplicate args by Xor and Equivalent

(If anybody would like to take this over, that would be fine with me.)
Why do you think \_still_relational is useful here?  `Relational(*args, evaluate=False)` - always is_Relational...