
theorem Th21:
  for S being TopStruct, l being Block of S for L being Subset of
  S st L=l holds L is strong
proof
  let S be TopStruct;
  let l be Block of S;
  let L be Subset of S;
  assume
A1: L=l;
  thus L is strong
  proof
    let x,y be Point of S;
    assume x in L & y in L;
    then {x,y} c= l by A1,ZFMISC_1:32;
    hence thesis;
  end;
end;
