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As far as I can see, the example partial order shown is not actually a series-parallel partial order: In the rectangle containing 7 elements, there is no line between the upper-left element and lower-right element. This prevents this 7-element set from being serially decomposed, and it obviously can't be parallel decomposed either. -- 85.81.37.198 ( talk) 15:49, 1 May 2019 (UTC) reply

Yes, there is a line missing. I'll see if I can find time to add it, if someone else doesn't get to it first. — David Eppstein ( talk) 16:58, 1 May 2019 (UTC) reply

Ok, done. You may need to reload to see the updated image. — David Eppstein ( talk) 02:24, 2 May 2019 (UTC) reply