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 Th15:
  Replace(<*p1, p2, p3*>, 1, q) = <*q, p2, p3*>
proof
  set f = <*p1,p2,p3*>;
  len f = 3 by FINSEQ_1:45;
  then Replace(f,1,q) = (f|(1-'1))^<*q*>^(f/^1) by Def1
    .= (f|0)^<*q*>^(f/^1) by XREAL_1:232
    .= <*q*>^(f/^1) by FINSEQ_1:34
    .= <*q*>^<*p2,p3*> by FINSEQ_6:47
    .= <*q*>^<*p2*>^<*p3*> by FINSEQ_1:32;
  hence thesis;
end;
