
theorem Th1:
  for A being non empty set, B being 1-element set st A c=
  B holds A = B
proof
  let A be non empty set, B be 1-element set;
  assume
A1: A c= B;
  ex s being Element of B st B = {s} by SUBSET_1:46;
  hence thesis by A1,ZFMISC_1:33;
end;
