If Semy uses only 2 colours can it be counterchanged?

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...