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 Th14:
  Replace(<*p1, p2*>, 2, q) = <*p1, q*>
proof
  set f = <*p1,p2*>;
A1: 2 -'1 = 1 + 1 -'1 .= 1 by NAT_D:34;
  2 <= len f by FINSEQ_1:44;
  then Replace(f,2,q) = (f|(2-'1))^<*q*>^(f/^2) by Def1
    .= (f|1)^<*q*>^(f/^len f) by A1,FINSEQ_1:44
    .= (f|1)^<*q*>^{} by RFINSEQ:27
    .= <*f.1*>^<*q*>^{} by FINSEQ_5:20
    .= <*f.1*>^<*q*> by FINSEQ_1:34
    .= <*p1*>^<*q*>;
  hence thesis;
end;
