Views can be specified by graphical correspondences between user data and a diagram of an abstract object.
Example: View a Christmas tree as a cone.
> (mkv 'cone 'xmas-tree)
(gldefun tg ((x xmas-tree))
(side-area (cone x)))
(LAMBDA (X)
(* 3.1415926535897931
(* (FIFTH X)
(SQRT (+ (EXPT (FIFTH X) 2)
(EXPT (CADDR X) 2))))))