I have tried writing something like the following, but it seems clumsy. Is there a better way to turn off vlines and hlines inside a table? One issue I have is that I can't tell the dimensions of the table from inside map-hlines, so I can't automatically know the end of the table.

#tablex(
    columns: (auto, auto, auto, auto, auto),
    auto-vlines: false,
    map-hlines: h => if (h.y == 0 or h.y == 2) { hlinex(stroke: none) } else { h },
    (), vlinex(), vlinex(), vlinex(), vlinex(), (),
    [a], [$u$], [$v$], [c], [d]
)