reserve D for non empty set,
  f for FinSequence of D,
  p, p1, p2, p3, q for Element of D,
  i, j, k, l, n for Nat;

theorem Th18:
  len Swap(f, i, j) = len f
proof
  dom Swap(f, i, j) = dom f by FUNCT_7:99;
  hence thesis by FINSEQ_3:29;
end;
