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- In a math textbook I found a statement that every empty set is equal.
- There are such sets that it is impossible for them not to be empty: set of all natural numbers between 10 and 11.
- There are such sets that it is possible for them not to be empty: all the people from the Earth, who are on the Mars now. In future it can be possible.
- Quesstion: can sets mentioned in 2 be equal to sets mentioned in 3. If yes, why not to take in account the difference examples above?