> Pokorny's 3. (*der-), 'tremble'; Pokorny's 1. *ter- is not a
separate root, nor a "Wechselform" (there is no such thing, in my
opinion), but merely an s-mobile form where the form _with_ *s- has
not been preserved.
>
I understand. There are no such things as Wechselforms, and they are
caused by s mobile.