Selecting subject headings and keywords is a chore for all metadata editors, who often leave these fields blank or incomplete—even when there are no guidelines and any word or phrase can be chosen. For example, tags are absent from the vast majority of citations in the social scholarly reference repository CiteULike. Libraries employ professional cataloguers and indexers to ensure consistent subject metadata in their records. Because this task is time-consuming, professionals and volunteers alike would welcome high-quality automatically generated suggestions for the main topics of a document.