    The function matching the i(regular expression) rules (tt(lex)) is a
member of the class hi(scanner)tt(Scanner). Since tt(Scanner) is derived from
tt(ScannerBase), it has access to all of tt(ScannerBase)'s protected members
that execute the lexical scanner's regular expression matching algorithm.


    Looking at the regular expressions themselves, notice that we need rules
to recognize comment, ti(#include) directives, and all remaining characters.
This all is fairly standard practice. When an tt(#include) directive is
sensed, the directive is parsed by the scanner. This too is common
practice. Our lexical scanner performs the following tasks:
    itemization(
    it() As usual, i(preprocessor directive)s are not
analyzed by a parser, but by the lexical scanner;
    it() The scanner uses a i(mini scanner) to extract the filename from the
directive, throwing a exception if this fails;
    it() If the filename could be extracted, processing switches to the next
stream, controlling for a maximum nesting depth.
    it() Once the end of the current file has been reached processing
automatically returns to the previous file, restoring the previous file name
and line number. The scanner returns 0 if all files have been processed.
    )
