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In set theory, we define integers as
0 = {}
1 = {0} = {{}}
2 = {0,1} = {{},{{}}}
3 = {0,1,2} = {{},{{}},{{},{{}}}}
4 = {0,1,2,3} = {{},{{}},{{},{{}}},{{},{{}},{{},{{}}}}}
...
Given a non-negative integer, output its set theory expression.
Order in sets doesn't matter. E.g. outputting 4 as {...