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 Th13:
  Replace(<*p1, p2*>, 1, q) = <*q, p2*>
proof
  set f = <*p1,p2*>;
A1: len f = 2 by FINSEQ_1:44;
  1 + 1 = len f by FINSEQ_1:44;
  then
A3: f/^1 = <*f.2*> by FINSEQ_5:30;
  Replace(f,1,q) = (f|(1-'1))^<*q*>^(f/^1) by A1,Def1
    .= (f|0)^<*q*>^(f/^1) by XREAL_1:232
    .= {}^<*q*>^<*p2*> by A3
    .= <*q*>^<*p2*> by FINSEQ_1:34;
  hence thesis;
end;
