http://www.omdoc.org/pubs/omdoc1.2.pdf の印刷ページ21（p.37）から：
Definition 3.2.5 (Monoid)
A monoid is a semigroup $`S = (G, \circ)`$ with an element $`e\in G`$, such that
$`e ◦ x = x`$ for all $`x ∈ G`$. $`e`$ is called a left unit of $`S`$.
Lemma 3.2.6
A monoid has at most one left unit.
Proof: We assume that there is another left unit $`f`$ . . .
This contradicts our assumption, so we have proven the claim. QED
注目すべき点：
- フォントや修飾
- 書き方の流儀
- 番号付け、名前付け
- 既存の定義の再利用
- "A monoid" という言い回し
- monoid, semigroup, element, left unit のような用語〈term〉
