Is it possible to specify that a defcustom can only take the value of a mode?
More specifically, I've got a variable that's a map from major modes to lists of strings, and I wants users to be able to configure this map. So conceptually I want something like this:
:type '(alist :key-type 'mode :value-type '(repeat string)))
Is this possible, or do I just need to use documentation?