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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def expr(self, token_stream, token):
 """Top level parsing function. An expression is <keyword> <OR> <expr> or <keyword> <SEP> <expr> or <keyword> """

        lval = self.keyword(token_stream, token, {})
        token = next(token_stream)
        if token[0] == 'OR':
            op = or_
            token = next(token_stream)
        elif token[0] == 'TEXT' or token[0] == 'COLOR':
            op = lambda l, r: list(flatten([l, r]))
        elif token[0] == 'SEPARATOR':
            op = lambda l, r: list(flatten([l, r]))
            token = next(token_stream)
        else:
            return [lval]
        rval = self.expr(token_stream, token)
        return op(lval, rval)