Set
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A Set is a concept from mathematics. It is a simple concept really. There is a bag, that can hold things.
What to do with sets
Element of
You can put different things into the bag. Later on, you ask whether a certain thing is in the bag. Mathematicians call this emement of. Something is an element of a set, if you find that thing in the respective bag.
Emptz set
Like a bag, a set can also be empty. The empty set is like the empty bag, it has no things in it.
Comparing sets
Finally, two sets can be compared. TRhis is done by looking at two different bags. If they contain the same things, they are equal.