A **monoid** is a structure `(S, *, z)` consisting of an associative binary operation `*` over some set `S`, which is closed under `*`, and which contains an identity element `z` for `*`. That is:
-<blockquote>
-for all `s1`, `s2`, `s3` in `S`:<BR>
-(i) `s1*s2` etc are also in `S`<BR>
-(ii) `(s1*s2)*s3` = `s1*(s2*s3)`<BR>
+<pre>
+for all `s1`, `s2`, `s3` in `S`:
+(i) `s1*s2` etc are also in `S`
+(ii) `(s1*s2)*s3` = `s1*(s2*s3)`
(iii) `z*s1` = `s1` = `s1*z`
-</blockquote>
+</pre>
Some examples of monoids are: