Customizing `blink-matching-paren-distance'

[martin rudalics <rudalics@gmx.at>]

(defcustom blink-matching-paren-distance (* 25 1024)
   "*If non-nil, is maximum distance to search for matching open-paren."
   :type 'integer
   :group 'paren-blinking)

The doc-string implies that this variable can be nil but customize does
not permit setting it to nil.  Moreover, the doc-string should say what
happens when this variable is nil, like

(defcustom blink-matching-paren-distance (* 25 1024)
   "*If non-nil, is maximum distance to search for matching open-paren.

If nil, search stops at the begin of the accessible portion of the buffer."
   :type '(choice (const nil) integer)
   :group 'paren-blinking)

Also, users should not be allowed to enter a negative value here, but I
don't know whether there's a standard way to do this.  Writing

   :type '(choice (const nil)
                  (integer
                   :match (lambda (widget value) (>= value 0))))

causes a misleading error message when a negative value is entered.

A simple customization type like `nonnegative-integer' would be probably
the right thing.  But this might lead to asking for `positive-integer',
`nonnegative-number', `positive-float', ...