A paradigm

As previously stated, the passive voice in English is formed by combining a form of the verb to be with the past participle of a transitive verb. Its overall structure and its contrast with the active voice is probably easier to see if they are displayed in a paradigm of traditional English verb tenses.
http://www.ntid.rit.edu/sea/processes/passive/grammatical/structure
why do we need "a" before "paradigm of..."?
thanks