declaring that two rules should not both be enabled
Major Section:  THEORIES

(theory-invariant (incompatible (:rewrite left-to-right)
                                (:rewrite right-to-left)))

General Form: (incompatible rune1 rune2)

where rune1 and rune2 are two specific runes. The arguments are not evaluated. Invariant is just a macro that expands into a term that checks that theory does not contain both runes. See theory-invariant.