Aggregates can't recurse
An aggregate summarises a finished relation. If an aggregate depends on itself, its meaning breaks down:
If bob is out, members is 1, and 1 < 2 lets him in.
But then members is 2, and 2 < 2 keeps him out.
No value of members agrees with itself: every count leads to a different count.
Like negation in a cycle, there is no fixed point to converge to, so Datamog rejects it: "Aggregate predicate 'members' cannot be recursive."