... shape.1
A coe is a shape where black and white could, if not prohibited from doing so, alternate back and forth lifting each other's pieces. The standard one and it's alternate are:

\includegraphics{coe.eps} \includegraphics{coe2.eps} 
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... boards2
Without these we could not tell if we were breaking the coe rule. 
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... unlikely3
NB: this is speculation. 
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... leaf.4
That sort of double occurrence would violate the coe rule. 
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