Function grid_output_datapath 

pub fn grid_output_datapath<B: Datapath>(
    b: &mut B,
    g: &Grid,
    which: usize,
) -> Option<B::V>

Lower output which (in all_outs order) through the datapath vocabulary b, resolving the array row-major so every tile’s north/west inputs are already lowered (the feed-forward invariant). None when any live tile has no propositional lowering.