People get misled by these toy examples with object hierarchies where the only
operation being done is `echo`. `echo` implicitly applies `` $ `` to all of its
arguments. Where's your `$` methods for `TextDoc` and `TodoDoc`? I don't see
them anywhere, and neither does the compiler, so it applies the `$` that _is_
in scope, which isn't a method, it's a proc, which only knows about the
compile-time type of its argument, and you gave it a `DocItem`, which doesn't
have `text` or `todo` fields. Define `$` _methods_ on your objects and it will
work.
