ossia and lyrics

[David Bobroff]

I've been tasked with altering a score I've prepared.  The score 
currently consists of a piano part with a vocal line and a contrabass 
obligato line.  As it stands, all is well.  Now I need to put in ossia 
passages for the voice.  I have managed to create ossia passages by 
using examples in the manual but I'm running into a problem with getting 
the lyrics to appear.  I want the ossia musical passages to appear above 
the original but I don't want lyrics under the ossia passage, only under 
the original.  I currently have the separate elements of the vocal 
material defined like this:

% Lyrics
soprano = \lyricmode { "soprano lyrics" }

% music
sop =  \relative c'' { the soprano's music }

% assembled lyrics and music
vocal = {
<<
    \new Voice = sop \relative {
<<
    \clef G
    \global
    \sop
>>
    }
    \addlyrics { \soprano }
>>
}

I'd like to define my ossia sections and put them together with the 
definitions I already have without doing too much violence to the work 
I've already done.  I was experimenting with the ossias (ossiæ?) from 
section 1.6.2 of the Notation Manual but ran into trouble with the 
lyrics not showing up and getting an error about; cannot find Voice 
'uniqueContext0' or something to that effect.  I suspect there is some 
detail involving nesting, contexts or something which is proving beyond 
my ability to locate and/or decipher in the docs.

Help?

-David