[PostScript]
Monoid: trait
  introduces
     __\circ __: T, T -> T
    unit: -> T
  asserts forall x, y, z: T
    (x \circ y) \circ z == x \circ (y \circ z);
    unit \circ x == x;
    x \circ unit == x
  implies Semigroup, Identity
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