Return a pretty formatted representation of self.
        Include additional debug details if `debug` is True.

def pretty(self, indent=1, debug=False):
        """
        Return a pretty formatted representation of self.
        Include additional debug details if `debug` is True.
        """
        debug_details = ''
        if debug:
            debug_details += '<isliteral=%r, iscanonical=%r>' % (self.isliteral, self.iscanonical)
        if self.isliteral:
            pretty_literal = self.args[0].pretty(indent=0, debug=debug)
            return (' ' * indent) + '%s(%s%s)' % (self.__class__.__name__, debug_details, pretty_literal)
        else:
            return super(NOT, self).pretty(indent=indent, debug=debug)