Lines

• ((draw-line viewport) p1 p2 color)
  draw-line takes a viewport descriptor. It returns a function that draws a line in the viewport connecting positions p1 and p2. The optional color argument defaults to black.
• ((clear-line viewport) p1 p2)
  clear-line takes a viewport descriptor. It returns a function that clears a line in viewport connecting positions p1 and p2.
• ((flip-line viewport) p1 p2)
  flip-line takes a viewport descriptor. It returns a function that flips a line in viewport connecting positions p1 and p2.