<pre>
- <b>B</b> -+ +--- <b>C</b> --+ +---- <b>D</b> -----+ +-- <b>E</b> --
| | | | | |
- F: -----→ G: -----→ K: -----→
- | | | | | η | | | ψ
+ F: ------> G: ------> K: ------>
+ | | | | | η | | | ψ
| | | | v | | v
- | | H: -----→ L: -----→
- | | | | | φ | |
+ | | H: ------> L: ------>
+ | | | | | φ | |
| | | | v | |
- | | J: -----→ | |
+ | | J: ------> | |
-----+ +--------+ +------------+ +-------
</pre>
φ[C2] ∘ η[C2] ∘ G(f) = J(f) ∘ φ[C1] ∘ η[C1]
</pre>
-Hence, we can define <code>(φ -v- η)[x]</code> as: <code>φ[x] ∘ η[x]</code> and rely on it to satisfy the constraints for a natural transformation from `G` to `J`:
+Hence, we can define <code>(φ -v- η)[\_]</code> as: <code>φ[\_] ∘ η[\_]</code> and rely on it to satisfy the constraints for a natural transformation from `G` to `J`:
<pre>
(φ -v- η)[C2] ∘ G(f) = J(f) ∘ (φ -v- η)[C1]
<pre>
(φ -h- η)[C1] = L(η[C1]) ∘ ψ[G(C1)]
- = ψ[H(C1)] ∘ K(η[C1])
+ = ψ[H(C1)] ∘ K(η[C1])
</pre>
Horizontal composition is also associative, and has the same identity as vertical composition.