a generative grammar is used to derive a string, rule order is intrinsically
predetermined by the availability of variables in the string under derivation.
This process is generally non-deterministic because there may be several
candidate rules. The idea of a
as suggested by Laske (1973a:365), is to impose an extrinsic ordering of rules
reflecting a certain manner in which the generation process is envisaged by the
composer. Alternate methods involve
(see reference manual §8.1).