
theorem Th17:
  for V being RealUnitarySpace, W being Subspace of V, u,v being
  VECTOR of V st u in W & v in W holds u - v in W
proof
  let V be RealUnitarySpace;
  let W be Subspace of V;
  let u,v be VECTOR of V;
  assume that
A1: u in W and
A2: v in W;
  - v in W by A2,Th16;
  hence thesis by A1,Th14;
end;
