>If so, I'm sure you can find facts which show it didn't
>happen. What are they?
Yiddish starts within Middle High German (or even something
post-MHG). Period.
I.e., after (not prior to) all transformations, sound shifts
and lexical innovations, peculiarities, that prompts analysts
to deem Middle and Southern ("Ober") German as such, and not
as OHG or Frisian or Old Norse or else. Without this history
of the German language, there ain't no Yiddish. (Two thousand
years ago Jews spoke Aramaic, Greek, Latin and Hebrew.)
George