reserve n for Element of NAT,
  p,q,r,s for Element of HP-WFF;

theorem Th14:
  (0,1) --> (1,0) is Permutation of 2
  proof
    set f = (0,1) --> (1,0);
A1: rng f = 2 by CARD_1:50,FUNCT_4:64;
    dom f = 2 by CARD_1:50,FUNCT_4:62;
    then f is Function of 2,2 by A1,FUNCT_2:def 1,RELSET_1:4;
    hence thesis by A1,FUNCT_2:57;
  end;
