N_ represents the set of positive integers.
The four basic sets are assigned to the identifiers
C_ during system initialization.
The set of positive integers, too, is assigned to the identifier
system initialization. It is not represented by a basic set but by
the intersection of
Z_ and the interval
contains— Test whether some object is a member
is(a in S) may be used.
convert— Convert a domain into a basic set
set2prop— Convert a set to a property