position of ossia staff

[Graham Percival]

I'd like to include an ossia (alternate part) in a piece.  The piece
normally has two staffs, and I'd like to have an alternate staff between
the two staffs.  However, Lilypond puts the alternate staff at the
bottom of the staffs:
aaaa
cccc
  bb
(see example)

I'd like it to look like this:
aaaa
  bb
cccc

Is this possible?

------
\score {
    \context GrandStaff = gs \notes <
      \context Staff=$aaa { a'1 a'
         <{a' a'}
          \context Staff=$bbb {b'1 b'}
         >}
      \context Staff=$ccc { c''1 c'' c'' c''}
>}
-----

Cheers,
- Graham