Apparently `{.pure.}` _can_ be attached to an object type, but the manual
explains its purpose very poorly. Essentially it is only used for inheritance,
specifically when you create a type and attach the `{.inheritable.}` pragma to
it. This makes an object which can be inherited from, similar to `RootObj` from
the standard library, but now you have two such roots. To provide runtime type
information in Nim each inheritable objects adds a hidden field holding the
type of the object. This can be queried with `of` so you can do `if myVar of
InheritedObjectType`. If you don't need runtime type information, and you'd
rather not have Nim generate this field for you, then you should use the
`{.pure.}` pragma. Otherwise it does nothing (not even throw an error). This
seems to be most commonly used for [interoperability with C++
code](https://nim-lang.org/docs/manual.html#exception-handling-imported-exceptions)
to mark a type which can be inherited from.
