Adding durations (for \after)

[Joel C. Salomon]

<https://lilypond.org/doc/v2.23/Documentation/notation/writing-rhythms#scaling-durations> 
says:

> Factors may also be added by using Scheme expressions evaluating to a
> number or musical length like `*#(ly:music-length music)`.

Is there an example of such addition handy?

The stripped-down example below works, but `\after 8*9' would be better 
written `\after #( <something involving 2. and 4.> )`.

The best I could glean from the documentation looks something like:

  \after #(ly:moment-add (ly:make-duration 2 1) (ly:make-duration 4 1))

... except that yields a type error because I’m creating durations not 
moments.

--Joel


    \version "2.23.80"

    music = \relative c'' {
      \time 6/8
          c8[ c   c   c   c   c]  |
      <<
        {
          b4.(        a)          |
        } \\ {
          g2.                     |
        }
      >>
    }

    dynamics = {
      \after 2. \>
      \after 8*9 \!
          s2.*2                   ||
    }

    \score { <<
      \new Staff \music
      \new Dynamics \dynamics
    >> }