chordmode :13 leaving out 11

[Rune Zedeler]

Manual says "thirds are added to the root until it reaches the specified 
number" which would be the correct thing to do.
But when specifying :13, the 11 is left out, so that the chord name 
printed is "9/add13" instead of "13"

%%% begin %%%
\version "2.10.25"
mus = \chordmode { c:13 }
<<
  \new ChordNames \mus
  \new Staff \mus
>>
%%% end %%%

-Rune