bibtex-autofill-entry is an interactive compiled Lisp function in `
Try to fill fields of current BibTeX entry based on neighboring entries.
The current entry must have a key. Determine the neighboring entry
(previous or next) whose key is more similar to the key of the current
entry. For all empty fields of the current entry insert the corresponding
field contents of the neighboring entry. Finally try to update the text
based on the difference between the keys of the neighboring and the current
entry (for example, the year parts of the keys).