Function ledger_frame 

fn ledger_frame(
    ledger: &mut BlockLedger,
    changed: &[Rectangle<i32, Physical>],
    (y, height): (i32, i32),
    dirty: bool,
    observed: bool,
)

One frame of a region’s ledger, the region at rows y to y + height: the rectangles of changed that reach it, in its own coordinates, read with whether it changed (dirty) and whether its hash saw it change (observed, BlockLedger::frame_reported).