def eval_extensions(self, value, name, option, format):
try:<EOL><INDENT>exts = self._extensions[name]<EOL><DEDENT>except KeyError:<EOL><INDENT>raise ValueError('<STR_LIT>' % name)<EOL><DEDENT>for ext in exts:<EOL><INDENT>rv = ext(self, value, name, option, format)<EOL>if rv is not None:<EOL><INDENT>return rv<EOL><DEDENT><DEDENT>
Evaluates extensions in the registry.  If some extension handles the
        format string, it returns a string.  Otherwise, returns ``None``.