Characterization of Metacategory via Equations

Theorem

Let $\mathbf C_0$ and $\mathbf C_1$ be collections of objects.

Let $\operatorname{Cdm}$ and $\operatorname{Dom}$ assign to every element of $\mathbf C_1$ an element of $\mathbf C_0$.

Let $\operatorname{id}$ assign to every element of $\mathbf C_0$ an element of $\mathbf C_1$.

Denote with $\mathbf C_2$ the collection of pairs $\tuple {f, g}$ of elements of $\mathbf C_1$ satisfying:

$\Dom g = \Cdm f$

Let $\circ$ assign to every such pair an element of $\mathbf C_1$.

Then $\mathbf C_0, \mathbf C_1, \operatorname{Cdm}, \operatorname{Dom}, \operatorname{id}$ and $\circ$ together determine a metacategory $\mathbf C$ if and only if the following seven axioms are satisfied:

 $\ds \Dom {\operatorname{id}_A} = A$ $\qquad$ $\ds \Cdm {\operatorname{id}_A} = A$ $\ds f \circ \operatorname{id}_{\Dom f} = f$  $\ds \operatorname{id}_{\Cdm f} \circ f = f$ $\ds \Dom {g \circ f} = \Dom f$  $\ds \Cdm {g \circ f} = \Cdm g$ $\ds h \circ \paren {g \circ f}$ $=$ $\ds \paren {h \circ g} \circ f$

where $A$ and $f, g, h$ are arbitrary elements of $\mathbf C_0$ and $\mathbf C_1$, respectively.

Further, in the last two lines, it is presumed that all compositions are defined.

Hence it follows that: