Every equation is preceded by the <?Pub Eqn> processing instruction (PI), which is visible only in the XML or SGML source. Prior to release 6.0, equations were parsed by the Equation Editor and required that this processing instruction be present before the equation markup. Beginning in release 6.0, that processing instruction is no longer required and equations are parsed like any other markup. Arbortext Editor continues to write the processing instruction before equations for backwards compatibility.