The window [offset, offset + count) of a list with adaptive bounds (FDA AList.subA parity, poll node). All three dependencies — the source list and the two bound scalars — are re-read on every read (dependency registration); the window and the positional diff run only when any of their versions moved, so clean reads allocate nothing and run no user code (library invariant 5). The build route of PollListSourceNode cannot gate: its closure may read additional adaptive inputs, so this node exists with the extra versions tracked explicitly.