How to specify the domain of a function

Domains can be specified by:
- intervals
- union of interval (what about other set operations on intervals)
- inequality relations, e.g. `x != 5`, `a < x < b`
- set relations, e.g. `x in RR`

What about multidimensional domains?

Let's start with saying that domains have be a relation involving the same variable for which the domain is being specified for.