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.