
theorem Th20:
for G being subset-closed non empty set holds {} in G
proof let G be subset-closed non empty set;
   consider z being object such that
A1: z in G by XBOOLE_0:def 1;
   reconsider z as set by TARSKI:1;
   {} c= z;
   hence {} in G by A1,CLASSES1:def 1;
end;
