-Gutter-visible specifiers are used to specify the visibility of a
-gutter. The values of the variables `default-gutter-visible-p',
-`top-gutter-visible-p', `left-gutter-visible-p',
-`right-gutter-visible-p', and `bottom-gutter-visible-p' are always
-gutter-visible specifiers.
-
-Valid gutter-visible instantiators are t, nil or a list of symbols.
-If a gutter-visible instantiator is set to a list of symbols, and the
-correspondong gutter specification is a property-list strings, then
-elements of the gutter specification will only be visible if the
-corresponding symbol occurs in the gutter-visible instantiator.