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
  Replace(<*p*>, 1, q) = <*q*>
proof
A1: 1 - 1 = 0;
  set g = <*>D;
  reconsider P = <*p*>^g as FinSequence of D;
A2: (<*p*>^g)/^1 = g by FINSEQ_6:45;
  1 <= len <*p*> by FINSEQ_1:39;
  hence Replace(<*p*>, 1, q) = (<*p*>|(1-'1))^<*q*>^(<*p*>/^1) by Def1
    .= (<*p*>|0)^<*q*>^(<*p*>/^1) by A1,XREAL_0:def 2
    .= <*q*>^(<*p*>/^1) by FINSEQ_1:34
    .= <*q*>^(P/^1) by FINSEQ_1:34
    .= <*q*> by A2,FINSEQ_1:34;
end;
