However, in most existing treebanks, [a ?] dependency or phrase structure is used [to annotate the sentences], depending on the syntax properties of the language.
I am not sure if I need article "a" before dependency or phrase structure, if it is possible I prefer not to have it. (structure can be uncountable/countable)
Update This paragraph from Wikipedia use them without article, why?! :
Some treebanks follow a specific linguistic theory in their syntactic annotation (e.g. the BulTreeBank follows HPSG) but most try to be less theory-specific. However, two main groups can be distinguished: treebanks that annotate phrase structure (for example the Penn Treebank or ICE-GB) and those that annotate dependency structure (for example the Prague Dependency Treebank or the Quranic Arabic Dependency Treebank).