Position Operations

A position is the place where a pixel goes in a viewport. The upper-left corner is pixel (0, 0) and the orientation of the position coordinates within a viewport is as follows:

•   (make-posn x y)
  make-posn takes two integers and returns a position with the specified x and y coordinates.
•   (posn-x p), (posn-y p)
  posn-x and posn-y return the x and y coordinates, respectively, of a position.
•   (posn? p)
  posn? reports whether its argument is a valid position.
•   ((get-pixel viewport) p)
  get-pixel returns the color of the pixel at position p in viewport; 0 denotes white and 1 denotes not white.