theorem Th5:
  <*p1,f1,q1*> is SFHT of D & <*q1,f2,p2*> is SFHT of D implies
  <*p1,PP_composition(f1,f2),p2*> is SFHT of D
  proof
    <*PP_inversion(q1),f2,p2*> is SFHT of D by NOMIN_3:19;
    hence thesis by NOMIN_3:25;
  end;
