\begin{definition}[Definition:Composable Morphisms]
Let $\mathbf C$ be a metacategory.
Let $f, g \in \mathbf C_1$ be morphisms of $\mathbf C$.
Then $f$ is said to be '''composable with $g$''' {{iff}}:
:$\Cdm f = \Dom g$
that is, {{iff}} the codomain of $f$ is the domain of $g$.
When the order of composition is to be made more explicit, one says that '''$\tuple {g, f}$ is a composable pair'''.
The collection of all such '''composable pairs''' in $\mathbf C$ is denoted $\mathbf C_2$.
\end{definition}