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class constructions to abstract properties of general sets is more common, as for example in
the definitions of cardinal and ordinal number below.
== Ordinal numbers ==
We say that two well-orderings <math>W_1</math> and <math>W_2</math> are <strong>similar</strong>
and write <math>W_1 \sim W_2</math> just in case there is a bijection f from the field of
<math>W_1</math> to the field of <math>W_2</math> such that <math>x W_1 y \leftrightarrow f(x)W_2f(y)</math> for all x and y.
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