The `\textbackslash` seems to produce the following warning:
```
Chapter 2.
(./chapters/arrows-practice.tex [5] [6]

LaTeX Font Warning: Font shape `OMS/cmtt/m/n' undefined
(Font)              using `OMS/cmsy/m/n' instead
(Font)              for symbol `textbackslash' on input line 119.

) (./chapters/freyd-cats.tex [7] [8]) [9]
```
Do you also get this?