Changes

===Solution===

===Solution===

[http://mathforum.org/kb/plaintext.jspa?messageID=6514666 Solution posted by Jon Awbrey, recasting the problem in logical graphs].

[http://mathforum.org/kb/plaintext.jspa?messageID=6514666 Solution posted by Jon Awbrey, using the calculus of logical graphs].

In logical graphs, the required equivalence looks like this:

In logical graphs, the required equivalence looks like this: