Add note on unexpected behaviour of Omit with unions

This behaviour is quite unexpected and made me lose many hours before realising it didn't work as I thought. I hope this helps future students.
Original code for `UnionOmit` by [Andrii Dieiev](https://github.com/IllusionMH), from: microsoft/TypeScript#39556 (comment)

Author: fcole90

src/content/9/en/part9d.md

@@ -1115,14 +1115,22 @@ interface HealthCheckEntry extends BaseEntry {
 }
 ```
 
-Now we only need to create the <i>OccupationalHealthCareEntry</i> and <i>HospitalEntry</i> types so we can combine and export them as an Entry type like this:
+Now we only need to create the <i>OccupationalHealthCareEntry</i> and <i>HospitalEntry</i> types so we can combine them in an union and export them as an Entry type like this:
 
 ```js
 export type Entry =
   | HospitalEntry
   | OccupationalHealthcareEntry
   | HealthCheckEntry;
 ```
+An important point concerning unions is that, when you use them with `Omit` to exclude a property, it works in a possibly unexpected way. Suppose we want to remove the `id` from each `Entry`, we could think of using `Omit<Entry, 'id'>`, but [it wouldn't work as we might expect](https://github.com/microsoft/TypeScript/issues/42680). In fact, the resulting type, would only contain the common properties, but not the ones they don't share. A possible workaround is to define a special Omit-like function to deal with such situations:
+
+```ts
+// Define special omit for unions
+type UnionOmit<T, K extends string | number | symbol> = T extends unknown ? Omit<T, K> : never;
+// Define Entry without the 'id' property
+type EntryWithoutId = UnionOmit<Entry, 'id'>;
+```
 
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