I learned all I needed to know about elm in History, and was nothing to do with rot, but more in HH territory*...
Mea culpa... I confused yew with elm**, yew is indeed used to perforate distant naughty people, but I did know about elm and its rotproof qualities in a historical context, it was used as the bottom boards of narrowboats.
* its use in 6ft poles to shoot sharp pointy sticks at other people with extreme rapidity.
** "Tree, three letters, contains the letter 'e'...