proc glm data=data_prg.glm5_14;
 class a b c;
 model x=a b c a*b a*c b*c ;
 contrast 'c1 vs c2/a1' c 1 -1 a*c 1 -1;
 contrast 'c1 vs c2/a2' c 1 -1 a*c 0  0 1 -1;
 contrast 'c1 vs c2/a3' c 1 -1 a*c 0  0 0 0 1 -1;
 contrast 'c1 vs c2/a4' c 1 -1 a*c 0  0 0 0 1 -1;
means a b c  a*c /bon;
run;
quit;