How to do this correctly?

[David Kastrup]

The following does what I _want_ but looks more like an accident than
anything else.

\new Voice \with { \consists "Ambitus_engraver" }
{ \key des\major \set keySignature = #'() des' d'' }

What I want is the ambitus to be engraved without signature.

Working with \override Ambitus #'keySignature #'() and similar does not
appear to have any effect.

Setting the keySignature of the context in this manner would not appear
to be sane and likely has unwarranted side effects on other constructs.

So how to do this better?

-- 
David Kastrup