(object? v) returns #t if v is a object, #f otherwise.
(class? v) returns #t if v is a class, #f otherwise.
(object-class obj) returns the class of the object obj.
(is-a? v class) returns #t if v is an instance of class (or of a class derived from class), and #f otherwise.
(subclass? v class) returns #t if v is a class derived from (or equal to) class, #f otherwise.
(ivar-in-class? name class) returns #t if class (or any of its superclasses) defines an instance variable name (where name is an identifier), #f otherwise.