Setoid.eq_of_mem_classes

∀ {α : Type u_1} {r : Setoid α} {x : α} {b : Set α},
  b ∈ r.classes → x ∈ b → ∀ {b' : Set α}, b' ∈ r.classes → x ∈ b' → b = b'

If x ∈ α is in 2 equivalence classes, the equivalence classes are equal.

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