handle let expressions inside terms by unfolding (instead of raising an exception),
added examples to test this feature
(benchmark Isabelle
:assumption (not (forall (?x1 Int) (?x2 Int) (or (< 2 (+ ?x1 ?x2)) (or (= (+ ?x1 ?x2) 2) (< (+ ?x1 ?x2) 2)))))
:formula true
)