Common lisp has this universal setter-macro, SETF, which can be used to set values to all sorts of places. In this case you would use (setf (vref chip x y

Common lisp has this universal setter-macro, SETF, which can be used to set values to all sorts of places. In this case you would use (setf (vref chip x y) new-value) to call the function (just like with AREF above). Using SETF has the advantage that you can define "modify macros" that read and write a place. For example, (incf (vref chip x y) delta) would increment the place by DELTA. Or (rotatef (vref chip x1 y1) (vref chip x2 y2)) would swap two pixels.