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## 12.4 Hierarchies

A second example of a category is SemiGroup, defined by: SemiGroup

SemiGroup(): Category == SetCategory with
"*": ($,$) -> $
"**": ($, PositiveInteger) -> $

This definition is as simple as that for SetCategory, except that there are two exported operations. Multiple exported operations are written as a pile, that is, they all begin in the same column. Here you see that the category mentions another type, PositiveInteger, in a signature. Any domain can be used in a signature.

Since categories extend one another, they form hierarchies. Each category other than Type has one or more parents given by the one or more categories mentioned before the with part of the definition. SemiGroup extends SetCategory and SetCategory extends both Type and CoercibleTo (OutputForm). Since CoercibleTo (OutputForm) also extends Type, the mention of Type in the definition is unnecessary but included for emphasis.

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