fn replaceFirst(text: Span<u8>, from: u8) -> u32 {
    var i: usize = 0
    while i < std.mem.len(text) {
        if text[i] == from {
            return i as u32
        }
        i = i + 1
    }
    return 999
}

pub fn main(world: World) -> Void raises {
    if replaceFirst(std.mem.span("abca"), 99) == 2 {
        check world.out.write("markov ok\n")
    }
}