(is? ob class) ob: any type class: any type returns a bool
#t if an object or RData belongs to the specified class.
For an object, its class is identified using a reference to a
class object. For an
RData, its RClass (if any) is identified using a symbol.
(is? obj Rect) (is? rdata 'Sprite)
The arguments to
is? may belong to any type. If the types are other than those noted
above, the function will succeed and return