def parse_arxiv_json(data):
    """ Parses the response from the arxiv_metadata collection in Solr 
    for the exact search on arxiv_identifier. It returns the published date of 
    the first version of the paper (i.e. if there are multiple versions, it 
    ignores the revision dates)"""
    docs = data['response']['docs']
    # There is only one result returned (num_rows=1 and that is the nature of the
    # data, there is only one paper with 1 arxiv identifier). As a JSON array is 
    # returned and we only want the first date, we take only the first element of the
    # array.
    return docs[0].get('published_date')[0]