Footnote 8 in cl.tex has index entries for list. They come out as footnote 30 in the index.
\footnote{%
More precisely, in the standard implementation of a list as a feature structure,
the type \textit{list}\istype{list} has two subtypes \textit{null}\istype{null} and \textit{non-empty-list},\istype{non-empty-list} and
\textit{non-empty-list} has the features \textsc{first}\isfeat{first}
and \textsc{rest},\isfeat{rest} where the value of \textsc{rest} is of type \textit{list}.
This means that the value of \textsc{rest} can itself have the feature \textsc{rest}.
See also \crossrefchapterw[\page \pageref{page-list-encoding}]{formal-background} on lists.
}
Footnote 8 in cl.tex has index entries for list. They come out as footnote 30 in the index.
Well, actually all type footnotes come out as 30.