Problems with segnos and codas

[Tim Rowe]

I have a coda immediately following a D.S., and have tried to mark it
up as follows:
  <fis a c>
    \override Score.RehearsalMark #'break-visibility = #begin-of-line-invisible
    \override Score.RehearsalMark #'self-alignment-X = #RIGHT
    \mark \markup {"D.S " \musicglyph #"scripts.segno"}
  |
    \override Score.RehearsalMark #'break-visibility = #begin-of-line-invisible
    \override Score.RehearsalMark #'self-alignment-X = #LEFT
    \mark \markup {\musicglyph #"scripts.coda" "Coda "} |
  g2 r2 |
(To be honest, they're just magic incantations I god from snippets,
and I have no real understanding of what they're doing).

That has two problems. The most significant is that it doesn't work at
all -- I get a warning that there are two simultaneous mark events,
and it junks the second, so I don't get the coda mark.

The one that you can't tell from the little bit that I've included
there is that the starting g of the coda should be tied over from the
"to coda" bar, so it should have a tie going into it.

Where am I going wrong? Is there an easy way to fix these problems?

-- 
Tim Rowe