Sentence:  Thus, underlying ‘agtus' was converted first into ‘āgtus' by the vowel lengthening rule, and then into ‘āktus' by the ancient persistent rule. This example has previously been interpreted as indicating that new rules can enter a <WSD>phonology</WSD> elsewhere than at depth I. 
Meaning of the word marked with <WSD> tags:  The way sounds function within a given language; a phonological system.