Return a single field that is the concatenation of field's and subfield's
values, separated by a colon, updating field if possible, or inferring the key
if only subfield is supplied.

If item is an instance of Reference, and there are any sub(book)title
fields, remove the subtitle field and  from the field's
value and insert a new field with that value; otherwise, return item unchanged.

Recurse into crossref fields to find all used citekeys

If item is an instance of Reference, and there is a colon in a (book)title
field and no existing (book)subtitle, remove the subtitle from the field's
value and insert a new field with that value; otherwise, return item unchanged.

Change the case of s to match the case pattern of prototype.
Returns s unchanged if no pattern can be inferred from prototype.

Return true if r is an instance of the Reference class

Remove all of the fields in reference that have a key that occurs in
matching-keys, which will be all lower-cased and converted to a set.

If field's value contains a colon, move the content after the colon into a new field
and return both; otherwise return a singleton vector of the original field

Convert each word in s to title case, using a simple regular expression for detecting words