Defining new contexts

[Jim Long]

I'm experimenting with defining and using custom contexts.

I have not succeeded in applying the example at:

http://lilypond.org/doc/v2.18/Documentation/notation/defining-new-contexts

to my tiny example below attempting the creation of a custom
ChordNames context.  The '\new XChords' line throws a warning:

GNU LilyPond 2.18.2
Processing `foo.ly'
Parsing...
Interpreting music...
Preprocessing graphical objects...
Interpreting music...
warning: cannot find or create new `XChords'
Preprocessing graphical objects...
Finding the ideal number of pages...
Fitting music on 1 page...
Drawing systems...
Layout output to `foo.ps'...
Converting to `./foo.pdf'...
Success: compilation successfully completed


How can I modify the second score's source to have the scores
engrave the same in the output file?

Thank you!

Jim


\version "2.18.2"

\score {
  \layout {
    \context {
      \ChordNames
        \override ChordName.font-size = #12
    } % context
  } % layout
  <<
    \new ChordNames \chordmode { c1 }
    \new Staff      \chordmode { c1 }
  >>
}


\score {
  \layout {
    \context {
      \name XChords
        \alias ChordNames
        \override ChordName.font-size = #12
        \type "Engraver_group"
    } % context
  } % layout
  <<
    \new XChords \chordmode { c1 }
    \new Staff   \chordmode { c1 }
  >>
}