What relationships are used to create subclasses/superclasses?

One nice aspect of pronto is that it seems able to read most ontologies using a consistent API, even though ontologies don't always follow the same standards. As seen in [these docs](https://github.com/althonos/pronto/blob/8e946ef746a7a2999d32e266ed1996e87659ad69/docs/source/updating.rst), it is possible to get the parents and children for a term like:

```python
# parents
term.superclasses(distance=1, with_self=False)
# children
term.subclasses(distance=1, with_self=False)
```

What relationships are traversed when detecting sub/super-classes? For example, would these OBO relationships contribute to sub/superclasses: `is_a`, `has_subclass`, `has_part`? What about the RDF relationship of `rdfs:subClassOf`? Does each ontology define what types of relationships (i.e. predicates) constitute sub/superclasses? Or does pronto use the same hardcoded relationships for every ontology?

Is there a way to configure which relationships are indicative of sub/superclasses?

I did see that `is_a` and `has_subclass` might be special as per:

https://github.com/althonos/pronto/blob/8e946ef746a7a2999d32e266ed1996e87659ad69/pronto/ontology.py#L450