(error? (def a 1)) -> false
a -> 1
(error? (def a 300)) -> false
a -> 300
(error? (def a "strings")) -> false
a -> "strings"
(error? (def a 1)) -> false
a -> (:quoted :symbols 123 :or "strings")
(error? (set! a 42)) -> false
a -> 42
(error? (set! b 99)) -> true