Cautionary accidentals in Ambitus

[Cameron Horsburgh]

Hi folks,

I've recently written a timpani part for a score I'm writing, and I thought I'd 
add an ambitus to the beginning of the part to show the necessary tuning (I 
know, it's not standard!)

The two notes I want the timps tuned to are aes and des, and the key is des 
major. However, because the key signature (which comes _after_ the ambitus) 
includes aes and des there are no flats on the ambitus. How do I add cautionary 
accidentals?

I notice in the programme reference that Ambitus_engraver has a setting for 
cautionary accidentals, but it doesn't tell me how to turn them on or off.

Here's a simple version of my code:

\version "2.9.4"

\score{

  \relative c {
    \clef bass
    \key des \major
    des aes des aes |
  }

  \layout{
    \context {
      \Voice
      \consists Ambitus_engraver
    }
  }
}
-- 

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Cameron Horsburgh

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