Issues when directives share the same title and no number #51

Hi,

It may not be very frequent, but it may be worth fixinf anyway.

I write:

```{prf:theorem} Title1
:nonumber:
Content1
```
```{prf:lemma} Title1
:nonumber:
Content2
```

And surprisingly, the first directive becomes
Theorem Lemma (Title1):
it stole "Lemma" from the second directive.
I guess this is because the "Theorem" and "Lemma" (and all other directives actually) are assigned to the first match.

The issue disappears when removing :nonumber

Nath