Re: find-file-hook as illustration of Custom problems

[Richard Stallman]

    > This is the wrong solution.  The right solution for variables such as
    > minibuffer-prompt-properties is to set up a valid expression for its
    > standard value.  It may be necessary to store the current default
    > value in a new variable 

    or use a `valid-expression' property on the variable 

I do not understand that suggestion.
The symbol `valid-expression' does not occur in cus-edit.el.
If you are proposing it as the way to represent some new feature,
I don't know what the new feature would do.