reserve
   a,b,c,x,y,z,A,B,C,X,Y for set,
   f,g for Function,
   V for SetValuation,
   P for Permutation of V,
   p,q,r,s for Element of HP-WFF,
   n for Element of NAT;

theorem Th9:
  for f being involutive Function of A,A holds f*f = id A
  proof
    let f be involutive Function of A,A;
A1: dom f = A by FUNCT_2:52;
    then rng f c= dom f;
    hence thesis by A1,Th7;
  end;
