Function plg_rt_b_plus_3 

#[unsafe(no_mangle)]
pub extern "C" fn plg_rt_b_plus_3(
    m: *mut Machine,
    x: u64,
    y: u64,
    z: u64,
    site_id: u32,
) -> i32

plus/3: Z = X + Y over integers; any single unbound is solved for.