Disabling customization items

[Bill Wohler <wohler@newt.com>]

How does one disable items in the customization interface?

Let's say I have the option foo with three features "a", "b", or "c".

    (defcustom foo "a" nil
      :type '(choice (const "a")
                     (const "b")
                     (const "c")))

Let's say feature "b" is not available on a system. I'd like to indicate that
that item is supported by my code, but not by the system. Thus, choice "b"
should be disabled.

Thus customization interface in 21.2 doesn't appear to support this. Has
anyone figured out how to do this?

-- 
Bill Wohler <wohler@newt.com>  http://www.newt.com/wohler/  GnuPG ID:610BD9AD
Maintainer of comp.mail.mh FAQ and MH-E. Vote Libertarian!
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