Re: Is {{}} a valid construct?

> Syntax errors again.

No, you will have to refocus your mind. Every set consists of elements which are infact sets. It is recursive. If you are telling me set theory does not include this recursion, set theory is incomplete.

The following two examples are equivalent because: apple = {red, stem, etc}
orange = {navel, pulp, etc}

U                           =   {{}, apple, orange}
NOT apple              => {{}, orange}
NOT orange            => {{}, apple}
NOT {}                    => {apple, orange}
                                   -or-
                                   {{}, apple, orange}

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