Sentence:  In order to account for this, we might propose to make the Prepositional Phrase an "optional" constituent of the Verb Phrase: this we could do by re-/  placing rule (28) (ii) by rule (40) below:/ (40)      VP &rarr; V AP (PP)/ (Note that a constituent in parentheses is, by <WSD>convention</WSD>, taken to be/  optional.) 
Meaning of the word marked with <WSD> tags:  A practice or procedure widely observed in a group, especially to facilitate social interaction; a custom.