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 Th16:
  Replace(<*p1, p2, p3*>, 2, q) = <*p1, q, p3*>
proof
  set f = <*p1,p2,p3*>;
A1: 2 -'1 = 1 + 1 -'1 .= 1 by NAT_D:34;
A2: len f = 2 + 1 by FINSEQ_1:45;
  len f = 3 by FINSEQ_1:45; then
  Replace(f,2,q) = (f|(2-'1))^<*q*>^(f/^2) by Def1
    .= (f|1)^<*q*>^<*f.3*> by A1,A2,FINSEQ_5:30
    .= (f|1)^<*q*>^<*p3*>
    .= <*f.1*>^<*q*>^<*p3*> by FINSEQ_5:20
    .= <*p1*>^<*q*>^<*p3*>;
  hence thesis;
end;
