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## 12.7 Axioms

In the previous section you saw the complete Axiom program defining axiom SemiGroup. According to this definition semigroups are sets with the operations * and **. SemiGroup

You might ask: ``Aside from the notion of default packages, isn't a category just a macro, that is, a shorthand equivalent to the two operations * and ** with their types?'' If a category were a macro, every time you saw the word SemiGroup, you would rewrite it by its list of exported operations. Furthermore, every time you saw the exported operations of SemiGroup among the exports of a constructor, you could conclude that the constructor exported SemiGroup.

A category is not a macro and here is why. The definition for SemiGroup has documentation that states:

Category SemiGroup denotes the class of all multiplicative semigroups, that is, a set with an associative operation *.

Axioms:

associative("*" : ($,$)->$) -- (x*y)*z = x*(y*z)

According to the author's remarks, the mere exporting of an operation named * and ** is not enough to qualify the domain as a SemiGroup. In fact, a domain can be a semigroup only if it explicitly exports a ** and a * satisfying the associativity axiom.

In general, a category name implies a set of axioms, even mathematical theorems. There are numerous axioms from Ring, for example, that are well-understood from the literature. No attempt is made to list them all. Nonetheless, all such mathematical facts are implicit by the use of the name Ring.

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