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Cladistics question

   I've been reading Weishampel's new textbook and I have a quick question:

   Let's say we have hypothetical clades A and B, with B nested within A.
Let's pretend for a moment that green and blue buttons are legitmate
characters that could be used for diagnosing these clades.  Clade B is
comprised of blue buttoned animals.  Everything else outside of clade B in
clade A is comprised as green buttoned animals.  Clade B can be diagnosed
using the blue button character because all Bs have blues, yes?  Can Clade A
be diagnosed using green buttons even though Bs have blues (assuming all
animals outside of A have red buttons)?

   Also, someone recently mentioned a stem vs. a node based clade.  Wot's
the difference?

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