  As shown in [28], one of the five isomorphism types of optimal binary
subspace codes of size 77 for packet length v=6, constant dimension k=3 and
minimum subspace distance d=4 can be constructed by first expurgating and then
augmenting the corresponding lifted Gabidulin code in a fairly simple way. The
method was refined in [32,26] to yield an essentially computer-free
construction of a currently best-known plane subspace code of size 329 for
(v,k,d)=(7,3,4). In this paper we generalize the expurgation-augmentation
approach to arbitrary packet length v, providing both a detailed theoretical
analysis of our method and computational results for small parameters. As it
turns out, our method is capable of producing codes larger than those obtained
by the echelon-Ferrers construction and its variants. We are able to prove this
observation rigorously for packet lengths v = 3 mod 4.
