pm#nothing
  type:  class__clas  classes differ from sets in three important respects: 1) classes are not assumed to be extensional, i.e. distinct classes might well have exactly the same instances, 2) classes typically have an associated `condition' that determines the instances of the class, e.g., the condition `human' determines the "class of humans" (note that some classes might satisfy their own condition (e.g., the "class of abstract things" is "abstract") and hence be instances of themselves), 3) the instances of a class may occur only once within the class, i.e. a class cannot contain duplicate instances

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