  The K-ring of symmetric vector bundles over a scheme X, the so-called
Grothendieck-Witt ring of X, can be endowed with the structure of a (special)
$\lambda$-ring. The associated $\gamma$-filtration generalizes the fundamental
filtration on the (Grothendieck-)Witt ring of a field and is closely related to
the "classical" filtration by the kernels of the first two Stiefel-Whitney
classes.
