How to "grey-out" buttons?

[Nils Rennebarth <nils ipe uni-stuttgart de>]

Is it possible to "grey-out" buttons to show that the corresponding function
currently isn't available? I couldn't find this in the FAQ.

Nils

-- 
Quotes from the net, featuring John Lapeyre[L] and Christopher F. Miller [M]:
M> I'm not sure what the right words are to describe Upside.  Last month they
M> mentioned the sendmail **web server** as an example of the failure of the
M> open source process....
L> Well, sendmail does a lousy job of serving webpages.

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