Sets

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:

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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 !=:

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