The 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 :value
property and can be any s-expression.
There are two variations of the const widget, namely
variable-item and function-item. These should contain a
symbol with a variable or function binding. The major difference from
the const widget is that they will allow the user to see the
variable or function documentation for the symbol.