Clever way to combine parenthesis-less functions and boolean expressions.
This isn't new, it's already done by POSIX shell script :)

    if ! check_stuff 123 "stuff" && is_ok 45 67; then
        ...
    fi

This could be done in SLUL too, but with keywords instead of
expressions:

    if not check_stuff 123 "stuff" and is_ok 45 67
        ...
    end

Or, using `where`:

    where c = check_stuff 123 "stuff"
    where o = is_ok 45 67
    if all_of c o
        ...
    end

    # related: when should `where` be evaluated?
    # - in the exact place it appears? (can be inefficient)
    # - at each "instantiation" (can be inefficient)
    # - lazilly, on first usage?
    #   (but perhaps with a requirement that the order cannot
    #    affect behavior)

Or, using `if_all`/`if_any`/`if_none`:

    if_all
        check_stuff 123 "stuff"
        is_ok 45 67
    then
        ...
    end

    # this is a form of a multi-block statement, with a expr-list-block
    # first and then a statement-block

Perhaps `all`, `any`, `none` should be keywords on their own,
so it can work with `while' loops?
(but it could be confusing/impossible with assignment to boolean variables)
(also, it needs to be able to "stop" at either `then`, `do`, ...)

    if all
        check_stuff 123 "stuff"
        is_ok 45 67
    then
        ...
    end


    while all
        check_stuff 123 "stuff"
        is_ok 45 67
    do
        ...
    end


    do
        ...
    while   # <--- ambuigity!!! :(
        check_stuff 123 "stuff"
        is_ok 45 67
    end