This class implements a simple state machine pattern, allowing you to
quickly build rules-based state machines in Perl. As a simple
implementation of a powerful concept, it differs slightly from an ideal
DFA model in that it does not enforce a single possible switch from one
state to another. Rather, it short circuits the evaluation of the rules
for such switches, so that the first rule to return a true value will
trigger its switch and no other switch rules will be checked. (But see the
strict attribute and parameter to new().) It differs from an NFA model in
that it offers no back-tracking. But in truth, you can use it to build a
state machine that adheres to either model--hence the more generic FSA
moniker.