The extended siddha-principle

P¯an.ini’s grammar includes several types of metarules which determine how its operational rules apply. Among them are “traffic rules” which constrain how rules interact with each other in grammatical derivations. These are typically formulated as designating a rule or class of rules asiddha “not effected” (or asiddhavat “as if not effected”) with respect to another rule or class of rules. For economy, the rules so designated are grouped into several sections, whose headings collectively declare them to be asiddha(vat). The biggest such section, under the famous heading 8.2.1 p ¯urvatr¯asiddham, extends from 8.2.1 through the end of the grammar (8.4.68), and is hence called the Trip¯ad¯ı ‘Three Sections’.