(* lots of negation *)
(negate (negate (negate 0)))
(* * and / are not used, * ) (but we use the names instead) *)
(plus 500 (minus 4 73))
(sum  30 (minus 4 5) (plus 200 202))
(sum 10 20 30 40 50 60 70 80 71)
(product 7 11 13 431)
(product 3 431 (sum 333000 666 1))
(negate (sum (negate 70) (negate 1) (times (product (sum (sum 1 1) 1))  (times 2 (negate (times 3 (times 4 5)))))))
(mean 432 433 431 430 429)
(plus 341 (mean 100 200 (mean 10 (mean 22 18 20)  30) 40))
