:: (Ord n, Floating n) => TurtleState n  -- ^ Turtle to modify
      -> TurtleState n  -- ^ Resulting turtle
penUp t
 | isPenDown t = t # makeNewTrail #  \t' -> t' { isPenDown = False }
 | otherwise   = t

-- |
penUp :: (Ord n, Floating n) => TurtleState n  -- ^ Turtle to modify
      -> TurtleState n
penUp t
 | isPenDown t = t # makeNewTrail #  \t' -> t' { isPenDown = False }
 | otherwise   = t
penUp t
 | isPenDown t = t # makeNewTrail #  \t' -> t' { isPenDown = False }
 | otherwise   = t