If Semy uses only 2 colours can it be counterchanged?

[karlwilcox]

At the moment we don't allow something like "azure semy de mullets vert a bend counterchanged" because we could have multi colour charges used in the semy (e.g. semy of mullets per pale or and vert), which can't be counterchanged.

However if we have just two colours, this should probably be allowed. Not straight forward though as we may to have to go poking through the parse tree...