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Intuitively: unordered container where all the values are unique, just like C++ std::set.
More precisely for set theory formalization of mathematics:
  • everything is a set, including the elements of sets
  • string manipulation wise:
    • {} is an empty set. The natural number 0 is defined as {} as well.
    • {{}} is a set that contains an empty set
    • {{}, {{}}} is a set that contains two sets: {} and {{}}
    • {{}, {}} is not well formed, because it contains {} twice