reserve A for non empty set,
  a,b,x,y,z,t for Element of A,
  f,g,h for Permutation of A;

theorem
  f\(id A) = f
proof
  thus f\(id A) = f*(id A)" by FUNCT_2:17
    .= f*(id A) by FUNCT_1:45
    .= f by FUNCT_2:17;
end;
