apply validation

```
See for example 
http://stackoverflow.com/questions/12979797/how-to-validate-a-rdf-with-your-rdf-
schema

Apply after transform removed the original data and wrote the output. Try to 
use the prefix mapping to know against which schema(s) validation should happen.

```

Original issue reported on code.google.com by `jo3....@gmail.com` on 16 Apr 2013 at 7:55
