### Missing symbol commands

You’re processing an old document, and some symbol commands such as `\``Box` and `\``lhd` appear no longer to exist. These commands were present in the core of LaTeX 2.09, but are not in current LaTeX. They are available in the latexsym package (which is part of the LaTeX distribution), and in the amsfonts package (which is part of the AMS distribution, and requires AMS symbol fonts).

`amsfonts.sty`
  fonts/amsfonts/latex (gzipped tar, browse)
`AMS symbol fonts`
  fonts/amsfonts/sources/symbols (gzipped tar, browse)

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=missssymb