Missing symbol commands
You're processing an old document, and some symbol commands such as
\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
Source: Missing symbol commands