Univ.UniverseSourceType of universes. A universe is defined as a set of level expressions. A level expression is built from levels and successors of level expressions, i.e.: le ::= l + n, n \in N.
A universe is said atomic if it consists of a single level expression with no increment, and algebraic otherwise (think the least upper bound of a set of level expressions).
Try to get a level out of a universe, returns None if it is an algebraic universe.
Get the levels inside the universe, forgetting about increments