  McLean's notion of Selective Interleaving Functions (SIFs) is perhaps the
best-known attempt to construct a framework for expressing various security
properties. We examine the expressive power of SIFs carefully. We show that
SIFs cannot capture nondeducibility on strategies (NOS). We also prove that the
set of security properties expressed with SIFs is not closed under conjunction,
from which it follows that separability is strictly stronger than double
generalized noninterference. However, we show that if we generalize the notion
of SIF in a natural way, then NOS is expressible, and the set of security
properties expressible by generalized SIFs is closed under conjunction.
