P_bibfile	-> bibfile (P_entry,P_entry*)
P_entry	-> entry P_article|P_inproceedings|P_misc
P_article	-> article P_author,P_title,P_journal,P_volume?,P_pages?,P_year,P_month?,P_url?
P_inproceedings	-> inproceedings P_author,P_title,P_booktitle,P_pages?,P_year,P_month?,P_location?,P_url?
P_misc	-> misc P_author,P_title,P_year,P_month?,P_school?,P_type?,P_location?,P_url?
P_author	-> author
P_title	-> title
P_journal	-> journal
P_volume	-> volume
P_pages	-> pages
P_year	-> year
P_month	-> month
P_url	-> url
P_school	-> school
P_type	-> type
P_location	-> location
P_booktitle	-> booktitle