4.2.2 Fields and field-based search

It is desirable to be able to partition any document in the database into parts (called fields), and label each part by a field name. Queries can be targeted not only to whole documents but also to specific fields. An important field might be ``equation'', another might be ``theorem'', and yet another might be ``chapter-title''. Hierarchical fields (i.e., a field could be made up of several fields, to any level of nesting), if available, give more flexibility to system designers and to search users.