my is-dot-cons is a bad function...

I wrote is-dot-cons to check if it is a dot cons written at Is normal cons? Is dot cons? - Shammerism, but it has a bug. This function is not considered the following cons.

("100" . '(10 20 30))

This is a dot cons, but cdr is not atom, so my is-dot-cons can't judge correctly. I thought there are any way to check cdr section correctly, but I can't find it.