This is the default subclass of Mail::SRS. An instance of this subclass
is actually constructed when new Mail::SRS is called.
Note that allowing variable separators after the SRS\d token means that
we must preserve this separator in the address for a possible reversal.
SRS1 does not need to understand the SRS0 address, just preserve it,
on the assumption that it is valid and that the host doing the final
reversal will perform cryptographic tests. It may therefore strip just
the string SRS0 and not the separator. This explains the appearance
of a double separator in SRS1<sep><hostname>=<sep>.
See Mail::SRS for details of the standard SRS subclass interface.
This module provides the methods compile() and parse(). It operates
without store, and guards against gaming the shortcut system.