def METHOD_NAME(self):
    """Compiles self.PATTERN into self.pattern.
    Subclass may override if it doesn't want to use
    self.{pattern,PATTERN} in .match().
    """
    if self.PATTERN is not None:
        PC = PatternCompiler()
        self.pattern, self.pattern_tree = PC.METHOD_NAME(self.PATTERN,
                                                             with_tree=True)