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With new-style classes, maybe this can be done by
subclassing string somehow.  There would be a subclass for
tainted strings and trying to do most things with them would
raise an exception.  With taint checking enabled, functions
like os.getenv and cgi.FieldStorage would make objects
containing tainted strings.  You'd untaint them by passing
them to re.search or re.match and pulling out the match
variables, like in Per.