New values, no guarantee

Arithmetic and string operations build values not in the input, so the earlier reasoning that the set of facts is finite no longer holds:

nat(0).
nat(X + 1) :- nat(X).   # 0, 1, 2, 3, ... forever

Each round produces a larger number, so the iteration never reaches a fixed point. Datamog does not stop you: this program runs until you stop it.