: is-even? ( Int -- Bool )
    dup 0 i.= [
        drop true
    ] [
        1 i.- is-odd?
    ] if
;

: is-odd? ( Int -- Bool )
    dup 0 i.= [
        drop false
    ] [
        1 i.- is-even?
    ] if
;

: test-mutual ( -- )
    4 is-even?
    test.assert

    4 is-odd?
    not test.assert

    7 is-odd?
    test.assert

    7 is-even?
    not test.assert
;
