Category of Monoids is Category
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Theorem
Let $\mathbf{Mon}$ be the category of monoids.
Then $\mathbf{Mon}$ is a metacategory.
Proof
Let us verify the axioms $(C1)$ up to $(C3)$ for a metacategory.
We have Composite of Homomorphisms on Algebraic Structure is Homomorphism, verifying $(C1)$.
We have Identity Mapping is Automorphism providing $\operatorname{id}_S$ for every monoid $\left({S, \circ}\right)$.
Now, $(C2)$ follows from Identity Mapping is Left Identity and Identity Mapping is Right Identity.
Finally, $(C3)$ follows from Composition of Mappings is Associative.
Hence $\mathbf{Mon}$ is a metacategory.
$\blacksquare$