A package mathsem for pdf-LATEX implements a way to provide semantic meaning to symbols, without adding a large syntactical burden to the specification of a mathematical expression. It uses a concept of ‘active comment’, allowing the ‘%’ 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. The words become content of a tooltip, that ‘pops-up’ by the symbol in a PDF document. The idea extends to: 1. allow multiple instances of the same symbol have distinct meanings; 2. attach semantics to macro-names as well as character symbols; 3. allow nested tooltip rectangles, for sub-expressions; 4. assign defaults to be attached to symbols and macros, at either global or local levels, to maintain consistency of meaning within extended portions of a document. It is planned to use the same syntactical constructions to provide words for spoken ‘alternative text’, in the context of fully-tagged, accessible, mathematical content within PDF documents Thanks to Michael Kohlhase for ideas suggesting such a package.