Nullary Predicate Syntax about modus HOT 3 OPEN

Just my personal opinion... but I actually prefer no parenthesis if we have to enforce either one or the other, since being able to just write thing :- other_thing is quite satisfying 😄

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A related thought: maybe we can solve the user error problem by making sure that we have clear error messages. When I tried a :- b() or a :- b::c() by mistake (not knowing this was not allowed) the error message did not provide any useful information. Maybe we should have a special case for this - check if there is () following a predicate, and specifically say something like "parenthesis should be omitted for nullary predicates"...?

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I am in favour of enforcing parentheses for nullary predicates. Although I don't mind the other options.

I think the relational algebra operators would all work the same way, just that we'd be working on {()} or {}. So to be consistent, enforce p().

Regardless, it would improve readability if we just stuck to one (instead of making it optional).

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