theorem
  for W1,W2 being strict Subspace of V, C1 being Coset of W1, C2 being
  Coset of W2 holds C1 = C2 implies W1 = W2
proof
  let W1,W2 be strict Subspace of V, C1 be Coset of W1, C2 be Coset of W2;
  ( ex v1 st C1 = v1 + W1)& ex v2 st C2 = v2 + W2 by Def12;
  hence thesis by Th88;
end;
