Description: Define the class of natural numbers, which are all ordinal numbers that
are less than every limit ordinal, i.e., all finite ordinals. Our
definition is a variant of the Definition of N of BellMachover p. 471.
See dfom2 for an alternate definition. Later, when we assume the
Axiom of Infinity, we show _om is a set in omex , and _om can
then be defined per dfom3 (the smallest inductive set) and dfom4 .

Note: the natural numbers _om are a subset of the ordinal numbers
df-on . They are completely different from the natural numbers NN
( df-nn ) that are a subset of the complex numbers defined much later
in our development, although the two sets have analogous properties and
operations defined on them. (Contributed by NM, 15-May-1994)