Minor cleanups for SML, TeX and .doc files

* `Ho_Net.empty_net` renamed to `Ho_Net.empty` (see 059e250f621b90e5a05f9f3f3c3305bc7722da0b for details)
* `first_tok` code removed (see e5cccb76f02564042172dfe01b1c382c6ea24c72 for details)
* whitespace cleanups in {.sig,.sml} files
* minor fixes in .tex and .doc files