Описание тега set-difference
The difference between two sets A and B consists of all elements that are in A but not in B.
Set difference is a mathematical operation defined on sets. It is also known as the relative complement of B with respect to A.
If we have two sets, say A and B, their difference A - B is the set of all elements that are in A but not in B.
Several programming languages have built-in features to remove a collection of elements from another collection of elements. When both collections are sets (meaning each element can occur at most once per collection and the order does not matter), this implements set difference.
Source: Wikipedia