theorem
  <*p,f,q*> is SFHT of D &
  <*q,g,r*> is SFHT of D & <*PP_inversion(q),g,r*> is SFHT of D
  implies <*p,PP_composition(f,g),r*> is SFHT of D
  proof
    PP_or(r,r) = r;
    hence thesis by Th24;
  end;
