Detailed Instructions: Given a sentence and two mentions from the text (arguments), indicate a phrase (a verb or noun phrase) that describes the relationship between the provided arguments.
Q: Sentence: 'Mr Obama has already sat down the Chairman of the Joint Chiefs of Staff and General David Petraeus , head of US Central Command since he was sworn in as president last week .', Argument/Subject 1: 'petraeus', Argument/Subject 2: 'us central command'
A:
be head of