const widget can contain any Lisp expression, but the user is
prohibited from editing it, which is mainly useful as a component of one
of the composite widgets.
The syntax for the
const widget is:
type ::= (const [keyword argument]... [ value ])
The value, if present, is used to initialize the
property and can be any s-expression.
There are two variations of the
const widget, namely
function-item. These should contain a
symbol with a variable or function binding. The major difference from
const widget is that they will allow the user to see the
variable or function documentation for the symbol.