biblatex and etex with active equal sign (=)

Question

Please why does etex.sty fail to load with active =?

\documentclass[a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage[turkish]{babel}
\catcode`\==13
\makeatletter
\bbl@activate{=}
\makeatother
\usepackage[style=authortitle-icomp]{biblatex}
\addbibresource{biblatex-examples.bib}

\begin{document}

\section*{The \texttt{authortitle-icomp} style}

This style combines the features of \texttt{authortitle-ibid} and
\texttt{authortitle-comp}. It will implicitly enable the
\texttt{sortcites} package option at load time. This style is
intended for citations given in footnotes.

\footcite{aristotle:rhetoric,averroes/bland,aristotle:physics,aristotle:poetics}
\printbibliography

\end{document}

I get the error:

! Undefined control sequence.
\globmarks ->\et@xglob 
                       6\marks \mathchardef 
l.240 \let\newmarks=\globmarks

Moving

\catcode`\=\string=13
\makeatletter
\bbl@activate{=}
\makeatother

into the document body works, but I am still interested in why etex won't load with active =. Beyond etex, biblatex can't process its options when = is active.

Answer 1

You have discovered why babel delays activating its shorthand characters when LaTeX is executing \begin{document}.

The character = is used in the syntax rules of TeX and in those cases it must have category code 12; so any assignment such as \let\a=\b will fail if = is active. In the specific case TeX finds

\let\newmarks=\globmarks

but the = is active, so what happens is that TeX makes \newmarks equivalent to the active = and then finds \globmarks, so it expands it because it's not part of an assignment where expansion is suppressed. Since the replacement text of \globmarks contains \et@xglob which is defined only later, the error message is issued. But even if \et@xglob had been defined earlier, the reading of etex.sty would have already been compromised.