theorem Th14:
  a,b '||' c,c
proof
  consider e,f,g such that
A1: [[e,f],[g,g]] = [[a,b],[c,c]];
A2: (e`2_3-f`2_3)*(g`3_3-g`3_3) - (g`2_3-g`2_3)*(e`3_3-f`3_3) = 0.F by Lm3;
  (e`1_3-f`1_3)*(g`2_3-g`2_3) - (g`1_3-g`1_3)*(e`2_3-f`2_3) = 0.F &
  (e`1_3-f`1_3)*(g`3_3-g`3_3) -
  (g`1_3 -g`1_3)*(e`3_3-f`3_3) = 0.F by Lm3;
  hence thesis by A1,A2,Th12;
end;
