{Semantic enrichment of mathematics using `active~comments'} {Ross Moore} {A package \pkg{mathsem} for pdf\LaTeX\ implements, on the \emph{input} side, a way to provide semantic meaning for symbols, without adding a large syntactical burden to the specification of a mathematical expression. It uses a concept of `active comment', allowing the `\code{\%}' character at the beginning of a new line to become an active token under highly-controlled circumstances. With a strictly defined syntax, words to express the semantic meaning of a variable (`$x$' say) can be associated with each occurrence of `$x$' in the expression following. On the \emph{output} side the words can become content for a tooltip, that `pops-up' near the symbol within a \PDF\ document. Or the words can be used for vocalisation by assistive technology of the meaning of the symbol, thereby enhancing accessibility for the \PDF\ document. This idea extends to: %\begin{enumerate}[1.] \item{1.} allowing multiple instances of the same symbol have distinct meanings; \item{2.} attaching semantics to macro names as well as character symbols; \item{3.} allowing nested tooltips, for subexpressions; \item{4.} assigning defaults to be attached to symbols and macros, at either global or local levels, to maintain consistency of meaning within extended portions of a given document. %\end{enumerate} \noindent I will present examples of use of this package, and discuss implications and issues that arise, both from the kind of syntax being employed and in the internal coding that was required. A particular application is to provide words for spoken `alternative text', in the context of fully-tagged, accessible, mathematical content within \acro{PDF/UA} documents.}