<mathematics> A finite von Neumann ordinal.
The von Neumann integer N is a finite set with N elements which are the von Neumann integers 0 to N-1. Thus
0 = {} = {}
1 = {0} = {{}}
2 = {0, 1} = {{}, {{}}}
3 = {0, 1, 2} = {{}, {{}}, {{}, {{}}}}
...
The set of von Neumann integers is infinite, even though
each of its elements is finite.
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