AtBeginEndPackage
From charlesreid1
First, obtain the style file from here: http://www-h.eng.cam.ac.uk/help/tpl/textprocessing/atbeginend.sty
Now install the style file (also see Cancel_Package#Using_Cancel_Package_in_LyX page on this wiki) by putting it into ~/Library/texmf/tex/latex (if you are on a Mac).