src/HOL/SMT/Examples/cert/z3_linarith_18

handle let expressions inside terms by unfolding (instead of raising an exception),
added examples to test this feature

(benchmark Isabelle
:extrafuns (
  (uf_1 Int)
 )
:assumption (not (<= (+ uf_1 1) (+ uf_1 (+ (* 2 (mod uf_1 2)) 1))))
:formula true
)