fn upper(c: u8) -> u8 {
    if c >= 97 && c <= 122 {
        return c - 32
    }
    return c
}

fn lower(c: u8) -> u8 {
    if c >= 65 && c <= 90 {
        return c + 32
    }
    return c
}

pub fn main(world: World) -> Void raises {
    let source: Span<u8> = std.mem.span("Zero")
    var uppered: [4]u8 = [0_u8; 4]
    var lowered: [4]u8 = [0_u8; 4]
    var i: usize = 0
    while i < std.mem.len(source) {
        uppered[i] = upper(source[i])
        lowered[i] = lower(source[i])
        i = i + 1
    }
    if uppered[0] == 90 && uppered[1] == 69 && (lowered[0] == 122 && lowered[1] == 101 && lowered[3] == 111) {
        check world.out.write("string case ok\n")
    }
}