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 Th30:
  i <> k & j <> k & 1 <= k & k <= len f implies Swap(f, i, j)/.k = f/.k
proof
  assume that
A1: i <> k & j <> k and
A2: 1 <= k and
A3: k <= len f;
A4: k in dom f by A2,A3,FINSEQ_3:25;
  k <= len Swap(f, i, j) by A3,Th18;
  then k in dom Swap(f, i, j) by A2,FINSEQ_3:25;
  hence Swap(f, i, j)/.k = Swap(f, i, j).k by PARTFUN1:def 6
    .= f.k by A1,Lm4
    .= f/.k by A4,PARTFUN1:def 6;
end;
