|   | 1 |  initial version  | 
Like this
sage: S = Subsets([1,..,5])
sage: G = [t for t in S if sum(t)<=3]
sage: Poset((G, lambda x,y: x.issubset(y)))
Finite poset containing 5 elements
And I am slightly tempted to say RTFM.
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                Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.