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Like this maybe (not checked)

sage: def cata(n): 
....:     S = Subsets(list(range(n))) 
....:     def rel(x, y):
....:        return len(x)==len(y) and all(u<=v for u, v in zip(sorted(x),sorted(y))) 
....:     return Poset((S, rel))                              
sage: cata(4)                                                                   
Finite poset containing 16 elements