transparent ledger lines?

[David Bobroff]

I would like to make ledger lines transparent and then turn them back
on.  I looked through regression and /input/test/ and didn't find what I
was looking for.  I saw in one regression example that individual grobs
can be offset but I would find it more useful in this instance to turn
the ledger lines off and then back on.  I even tried \property
Voice.LedgerLines \override #'transparent = ##t and got a notice that
"LedgerLines" was not a grob.

-David Bobroff