reserve s for set,
  i,j for Nat,
  c,c1,c2,c3 for Complex,
  F,F1,F2 for complex-valued FinSequence,
  R,R1,R2 for i-element complex-valued FinSequence;

theorem
  -(F1 - F2) = F2 - F1
proof
  thus -(F1 - F2) = -F1 + --F2 by Th10
    .= F2 - F1;
end;
