Z_, or equivalently solvelib::BasicSet(Dom::Integer), represents the set of integers.
The four basic sets are assigned to the identifiers Z_, Q_, R_, and C_ during system initialization.
The set of positive integers, too, is assigned to the identifier N_ during system initialization. It is not represented by a basic set but by the intersection of Z_ and the interval Dom::Interval(, infinity).
The domain of basic sets know about the basic arithmetical and set-theoretic functions:
J:=Dom::Interval(3/2, 21/4): Z_ intersect J