The set of all non-negative integers, {0, 1, 2, ...}, written ZZ*.
There is no set called "the natural numbers" here, because the name means
this set to some authors and PositiveIntegers to others
(MathWorld recommends these two names over "natural number" for that reason,
https://mathworld.wolfram.com/N.html). A subset of Integers, and
like every special set a Domain a node can be declared over:
domain(x, ZZ*) is NaN at a negative integer.
https://github.com/asc-community/AngouriMath/issues/1409