If REAL-BINDING is a symbol and has a non-`nil’ `menu-enable’
property, that property is an expression that controls whether the menu
item is enabled. Every time the keymap is used to display a menu,
Emacs evaluates the expression, and it enables the menu item only if
the expression’s value is non-`nil’. When a menu item is disabled, it
is displayed in a “fuzzy” fashion, and cannot be selected.
How do I get the property of a symbol?