theorem Th7:
  a in A" iff a" in A
proof
  a"" = a & A"" = A;
  hence thesis;
end;

Lm1: A c= B implies A" c= B"
proof
  assume
A1: A c= B;
  let a be object;
  assume a in A";
  then ex g being Element of G st a = g" & g in A;
  hence thesis by A1;
