Macro erdos::make_callback_builder

macro_rules! make_callback_builder {
    (($rs_head:expr), (), $state:expr) => { ... };
    (($rs_head:expr), ($($ws:expr),*)) => { ... };
    (($($rs:expr),+), ($($ws:expr),*), $state:expr) => { ... };
    (($rs_head:expr, $($rs:expr),*), ($($ws:expr),*)) => { ... };
}

Makes a callback builder that can register watermark callbacks across multiple streams.

Note: an internal macro invoked by add_watermark_callback.