I didn't find anything like this in a Search of the Forums, so I'm posting little command I worked up, RestoreXline.lsp with its ReX command.
Say you had an Xline for construction/reference purposes that has somehow gotten Trimmed or Broken, so that it's now a Ray or even just a Line, and you're too far past when that happened to fix it by Undoing. Call up ReX, and pick it, and it will restore or repair or fix it, back to being an Xline, that is, extend it infinitely in both directions again. [Or you can also use it to just convert any Line or Ray, even if it wasn't originally an Xline, into one.] See further comments at the top of the file.
Kent Cooper, AIA 
