theorem
s<>TheNorSymbOf S & s<>TheEqSymbOf S implies s in OwnSymbolsOf S
proof
set O=OwnSymbolsOf S, R=RelSymbolsOf S, E=TheEqSymbOf S, X=R\O,
N=TheNorSymbOf S, SS=AllSymbolsOf S; assume s<>N & s<>E;then
not s in {N} & not s in {E} by TARSKI:def 1; then not s in {N}\/{E}
by XBOOLE_0:def 3;
hence thesis by XBOOLE_0:def 5;
end;
