31. Starting at the point (1/2,1/2) applying either rule **3**,
(x,y)->(x/2,y/2), or rule **1**, (x,y)->(x/2 + 1/2,y/2 + 1/2), gives a point on
the line between corners 1 and 3. In general, starting with a point on this line and
applying either rule **1** or rule **3** gives another point on this line.
So starting on the line and applying just these rules generates only points on this
line. (What happens if we start with a point not on this line, but still apply
only rules **1** and **3**?)

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