overriding methods implemented internally

It appears that one can override methods implemented internally, but can't use them.

```
toList Matrix := M -> flatten entries M
M = matrix{{1}}
class M
toList M

stdio:4:1:(3): error: expected list, sequence, or set
```

Start by running the Macaulay2 session shown in the issue and compare the behavior of the internally implemented method with the overriding method. Trace method lookup and the handling of `toList`, `flatten`, and `entries` to identify where the call becomes unusable. Done means the demonstrated override can be called successfully and the existing example no longer produces the reported type error.

Written by the indexing model from the issue text.