theorem Th6:
  <*p1,f1,q1*> is SFHT of D & <*q1,f2,q2*> is SFHT of D &
  <*q2,f3,p2*> is SFHT of D implies
  <*p1,PP_composition(f1,f2,f3),p2*> is SFHT of D
  proof
    assume that
A1: <*p1,f1,q1*> is SFHT of D & <*q1,f2,q2*> is SFHT of D and
A2: <*q2,f3,p2*> is SFHT of D;
A3: <*PP_inversion(q2),f3,p2*> is SFHT of D by NOMIN_3:19;
    <*p1,PP_composition(f1,f2),q2*> is SFHT of D by A1,Th5;
    hence thesis by A2,A3,NOMIN_3:25;
  end;
