Problem 103 (difficulty: 7/10)

Show an example of an associative operation \(\displaystyle \circ:\mathcal P(\R)\times\mathcal P(\R)\to\mathcal P(\R)\) for which the union operation is left distributive but not right distributive. (Here \(\displaystyle \mathcal P(\R)\) denotes the set of all subsets of the real line \(\displaystyle \R\).)

Give me another random problem!