The tt(Digit) and tt(Letter) coroutines handler classes are implemented
like tt(Start). Like tt(coStart), which continues its execution when a
non-digit and non-letter character is received, tt(coDigit) continues for as
long as digit characters are received, and tt(coLetter) continues for as long
as letter characters are received.

As we've seen in section ref(FSACORO) the implementation of tt(coDone) is a
bit different: it doesn't have to do anything, and the tt(coDone) coroutine
simply ends at its tt(co_return) statement. Coroutine execution (as well as
the FSA program) then ends following tt(main's g_start.go()) call.

The complete implementation of the coroutine-based FSA program is available in
the Annotation's distribution under the directory
tt(yo/coroutines/demo/fsa). 
