bib matches a bib
element
* matches
any element
/ matches
the root element
/bib matches
a bib element under root
bib/paper matches
a paper in bib
bib//paper matches
a paper in bib, at any depth
//paper matches
a paper at any depth
paper|book matches
a paper or a book
@price matches a price
attribute
bib/book/@price matches
price attribute in book, in bib
bib/book/[@price<55]/author/lastname matches