TeX glue is a metaphor invented by Prof Knuth and doesn't hark back
to manual typesetting days.

\begin{displaymath} ... \end{displaymath} is defined to use \[ ... \]
(The information in the cited source is incorrect when it states that
displaymath should never be used with amsmath.sty.)

(Thank you to Boris Veytsman for pointing out the above errors.)

Error reported through "report a typo" form:

Section 5.6: \begin{bibliography}{1} should be \begin{thebibliography}{1}