reserve D for non empty set;
reserve s for FinSequence of D;
reserve m,n for Element of NAT;

theorem Th17:
  for n be non zero Element of NAT,
  s,t be Element of n-tuples_on BOOLEAN
  holds Op-XOR(Op-XOR(s,t),t) = s
  proof
    let n be non zero Element of NAT,
    s,t be Element of n-tuples_on BOOLEAN;
    now let j be Nat;
      assume A1: j in Seg n;
      thus (Op-XOR(Op-XOR(s,t),t)).j
      = (Op-XOR(s,t)).j 'xor' t.j by A1,Def4
      .= (s.j 'xor' t.j ) 'xor' t.j by A1,Def4
      .= s.j by XBOOLEAN:72;
    end;
    hence thesis by FINSEQ_2:119;
  end;
