alternative within repeat does not work

[Jim Cline]

Hello, I would like to have an alternative within a repeat block, as 
described in the documentation,
https://lilypond.org/doc/v2.23/Documentation/notation/long-repeats#other-variation-in-repeated-sections
under "Other variation in repeated sections".  I don't need to work hard 
to create a minimal (not)working example, since the one given right there 
in the documentation gives me the same errors as in my full score:

\version "2.20.0"
\fixed c'' {
  \repeat volta 2 {
    R1
    \alternative {
      \volta 1 { c1 }
      \volta 2 { d1 }
    }
    R1
  }
}

The errors are:

error: syntax error, unexpected \alternative
    \alternative {
error: not a note name: volta
      volta 1 { c1 }
error: not a note name: volta
      volta 2 { d1 }
error: errors found, ignoring music expression
\fixed c'' {
fatal error: failed files: 
"/tmp/frescobaldi-w92qas0f/tmpfkzkv4p5/document.ly"

regards, Jim