An external module in the Archiving and Interchange DTD Suite that is first set up in the Module to Name the Modules (%modules.ent;) and is then called from the MathML setup module %mathmlsetup.ent; to include the MathML elements in the tag set for the DTD.
<!ENTITY % mathml2.dtd PUBLIC "-//W3C//DTD MathML 2.0//EN" "mathml2.dtd">
modules.ent