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