reserve S for 1-sorted,
  i for Element of NAT,
  p for FinSequence,
  X for set;

theorem Th21:
  bspace(X) is Abelian
proof
  let x,y be Element of bspace(X);
  reconsider A = x, B = y as Subset of X;
  x+y = B \+\ A by Def5
    .= y+x by Def5;
  hence thesis;
end;
