These represent primitive control sequences that are converted directly to Boxes or Lists containing
basic Unicode content, rather than structured XML, or those executed for side effect during digestion in
the LaTeXML::Core::Stomach, changing the LaTeXML::Core::State. The results of invoking a
"LaTeXML::Core::Definition::Primitive", if any, should be a list of digested items ("LaTeXML::Core::Box",
"LaTeXML::Core::List" or "LaTeXML::Core::Whatsit").
It extends LaTeXML::Core::Definition.
Primitive definitions may have lists of daemon subroutines, "beforeDigest" and "afterDigest", that are
executed before (and before the arguments are read) and after digestion. These should either end with
"return;", "()", or return a list of digested objects (LaTeXML::Core::Box, etc) that will be contributed
to the current list.