`<milestone rend="paragraphos" unit="undefined"/>` gets round-tripped to `----`

Example:

```
<milestone rend="paragraphos" unit="undefined"/>
```

Becomes:

```
----
```

Files which would exhibit this phenomenon with an idempotent Leiden+ transform:

```
DDB_EpiDoc_XML/c.pap.gr/c.pap.gr.2.1/c.pap.gr.2.1.15.xml
DDB_EpiDoc_XML/p.oxy/p.oxy.2/p.oxy.2.261.xml
DDB_EpiDoc_XML/o.krok/o.krok.1/o.krok.1.89.xml
```

See also #3.
