The outermost braces form a <singleton set>, regardless of how its one element is nested. Its <cardinality> is therefore \b[$1$]. A set containing the <empty set> is different from the empty set itself.

Solution

Past exam of the year 1 of the computer science course of the University of Oxford / 2017 / Discrete mathematics, continuous mathematics and probability / 1 / c / i / Solution

 Solution (source code)