toplevel "A";
"A" and "B" "B'" "C" "C'";
"B" and "D" "E";
"B'" and "D'" "E'";
"C" or "F";
"C'" or "F'";
"D" lambda=0.5 dorm=0;
"E" lambda=0.5 dorm=0;
"D'" lambda=0.5 dorm=0;
"E'" lambda=0.5 dorm=0;
"F" lambda=0.5 dorm=0;
"F'" lambda=0.5 dorm=0;