If you want to give any type a "null" value, you could use
[`std.typecons.Nullable`](https://dlang.org/library/std/typecons/nullable.html).
At LEAST for some things with currency types like prices which cannot be zero because 0 makes no sense for a price:
Nullable!(T, nullValue) is more storage-efficient than Nullable!T because it does not need to store an extra bool.
This could come handly. We'll see.
