BGs with an odd number of bases.

[sagji]

Another point. If the only way a BG can feed bases into melee is with an expansion, or a contraction, the diagram on page 73 "Feeding more bases into melee 1" is in violation of the rule. It shows a file "Moving to a position where it can contribute to the combat" by shifting across the entire battlegroup. This is neither a contraction nor an expansion as the frontage remains the same. If the only ways that a base can be moved to contribute to combat is with an expansion or contraction, BGs which count only the front rank for melee and which are in a single line could never shift troops from one end to the other.

Actually it is an example of an expansion - the BG is expanding on the right, the fact that it is contracting on the left doesn't prevent this - there is a restriction that it can't contract by more than one base width on any side.