!use defaults -> io -> println.
!use defaults -> effects.
!use defaults -> math -> +.
!use builtin -> block.
!use builtin -> return.
!entrypoint <- main.
(main) -> Int ? (effects -> IO).
main -> match (x <- println "Hello World") (
    (x >= 0) -> 0,
    (x < 0)-> 1,
)