From: David Tran <email55555@...>
Date: 2006-12-08T01:56:26+09:00
Subject: Re: [SUMMARY] Turtle Graphics (#104)

> Most of those should be obvious implementations.  The surprise, if any, comes
> from the fact that pen_down() puts a point on the track.  This makes sense
> though, if you think about it.  If you touch a pen to a piece of paper you have
> made a mark, even though you have not yet drawn a line.  The Turtle should
> function the same way.

Well, using this way, it may create "invalid" segment.

By definition:
track   ::= [segment, segment, ...] # drawing data
segment ::= [point, point, ...]     # points to be joined by line segments
point   ::= [x, y]                  # pair of floats

I was thinking a "no-empty" segment must have at least 2 points.
If we allow pen_down to make a mark, it may create invalid segment.
This test code shows:

home
pd   # mark [0,0]
pu
fd 100
pd
right 90
fd 100
pu
p track # ==> [[[0.0, 0.0]], [[0.0, 100.0], [100.0, 100.0]]]

so [[0.0, 0.0]] is not a valid segment, it is just a "mark".
And the Tk-viewer even not show it as a "point".
( for drawing the point [0,0], it maybe a segment like [[0,0],[0,0]] ? )

Anyway, in case you write your own viewer,
if we accept "mark", you need to test the invalid segment.