theorem Th9:
  a,b '||' c,d iff ([[a,b],[c,d]] in 4C_3(F) &
  ex e,f,g,h st [[a,b],[c,d]] = [[e,f],[g,h]] &
  (e`1_3-f`1_3)*(g`2_3-h`2_3) - (g`1_3-h`1_3)*(e`2_3-f`2_3) = 0.F &
  (e`1_3-f`1_3)*(g`3_3-
h`3_3) - (g`1_3-h`1_3)*(e`3_3-f`3_3) = 0.F &
   (e`2_3-f`2_3)*(g`3_3-h`3_3) - (g`2_3-h`2_3)*(e`3_3-f`3_3) =
  0.F)
by Def8;
