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
  1 <= i & i <= len f implies Replace(f, i, p)/.i = p
proof
  assume 1 <= i & i <= len f;
  then i in dom f by FINSEQ_3:25;
  hence thesis by FUNCT_7:36;
end;
