One point which is not clear to me, is when should the start condition
be resetted to INITIAL.
What I would achieve is to start the parsing of an input stream always
in the INITIAL start condition, and I expected it would be done by a
call to yyrestart(), but I realize it is not.
For the moment I reset it in every action which interrupts the parsing
(at end of stream, or when an unrecoverable error is found).
Are there a better way to do it?