Helpful error message:

[dax2]

This errormessage was most helpful:

Don't you want polyphonic voices instead?:

Yes I do! 

A quick glance in the user manual UNIFIED index:
Polyphony, and then I could see that I should add two backslashes, \\

So the parser seems to have some nice way to analyze the input,
wonder how it is done. The gis occurs as a whole note in the first
voice. No grudges(from the parser) after I wrote:

   %% bar 55
         <<{gis,1} \\
         {\times 2/3{gis,8(fis b}  \times 2/3{d'   b   fis )}
          \times 2/3{gis,8(d fis}  \times 2/3{b  fis    d  )} |
}>>
 


Regards
Donald Axel




Parsing...
Interpreting music... [8][16][24][32][40][48]
sonate-01.ly:454:23: warning: Adding note head to incompatible stem (type = 1):
         {\times 2/3{gi
                       s,8(fis b}  \times 2/3{d'   b   fis )}

sonate-01.ly:454:23: warning: Don't you want polyphonic voices instead?:
         {\times 2/3{gi
                       s,8(fis b}  \times 2/3{d'   b   fis )}
[56][64][68]
Preprocessing graphical objects... 
Calculating line breaks... [3][6][9][12][15][18][21][24][27][30][33][36][39][42]



-- 
dax2-tele2adsl:dk -- http://d-axel.dk/  Donald Axel