Automatically generated prefix alias can be just numbers

Example from here https://andrefs.github.io/chilon_rs/dbpedia/output.ttl:

```
<#164> <#namespacePrefix> <http://164.100.47.194/> .
<#2009-2017> <#namespacePrefix> <https://2009-2017.state.gov/> .
<#aat> <#namespacePrefix> <http://vocab.getty.edu/aat/> .
(...)
```