There are cases when you may want to return some special
value instead of an empty set. This is possible to do
with the coalescing operator - ??. This operator
returns the first operand if it's not empty and the
second operand otherwise:
Similarly there are versions of the comparison operators
?= and ?!= that allow comparing to an empty set.
These operators always return a boolean value with the
rule that an empty set is equal to another empty set and
nothing else (other values work the same as for = and
!=). Try it out and compare to plain = and !=:
