theorem Th28:
  for x being Element of bspace(X) holds (1_Z_2)*x = x
proof
  let x be Element of bspace(X);
  reconsider c = x as Subset of X;
  (1_Z_2)*x = (1_Z_2) \*\ c by Def6
    .= c by Def4;
  hence thesis;
end;
