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¶ ⦊
    ‖ The principal concept of set theory, the one that in
      completely axiomatic studies is the principle primitive
      (undefined) concept, is that of ‹belonging›. ⦉

    ‖ If $x$ belongs to $A$ ($x$ is an element of $A$, $x$
      is contained in $A$), we shall write
      ◇ ⦊
        ‖ x ∈ A. ⦉
      ⦉⦉

    ‖ This version of the Greek letter epsilon is so often
      used to denote belonging that its use to denote
      anything else is almost prohibited. ⦉

    ‖ Most authors relegate $ϵ$ to its set-theoretic use
      forever and use $ε$ when they need the fifth letter of
      the Greek alphabet. ⦉
  ⦉