Former pull #18: Major turtle changes: We now have turtle for: (1) contributors for images, figures, chapters, and reports, (2) turtle formalizing the person-organization-publication relationship, and (3) turtle formalizing the image-dataset-activity relationship. Additionally, (4) modified turtle to reflect report relationships/prov of "citedBy" and "cites"

I think  $file->identifier should be used here -- this is the permanent identifier for the file.  SHA1 is a checksum.