canonicalization problem with presubscript and primes

This comes from a CMU braille test (4.3.4 example 3): ${}_{++}z^{''}$

The TeX code produces:
```

```
The WIRIS editor produces (I tried prescripts first and primes first, both came out the same):
```

```

Canonicalization produces:
```

```
for the TeX.

The primes should be part of the `mmultiscripts` so the CMU rules can order the output correctly (primes after the base, then the presubscripts).

For the WIRIS encoding, the canonicalization does it wrong the opposite way:
```

```
Here there is an `msub` with an empty base inside the `mmultiscripts`.