Re: Is {{}} a valid construct?

Please ignore prior example, the following is more accurate as apple and orange are infact sets (and so are red, stem, orange, pulp, etc recursively)

U                           =   {{}, {red, stem}, {orange, pulp}}
NOT {red, stem}      => {{}, {orange, pulp}}
NOT {orange, pulp}  => {{}, {red, stem}}
NOT {}                    => {{red, stem}, {orange, pulp}}
                                  -or-
                                  {{}, {red, stem}, {orange, pulp}}