support hathitrust.org in xoclcnum service

xOCLCNUM has a less-used, but I think very useful feature of limiting search scope to a collection, we call it search in library feature. The goal is to limit FRBR expansion to a smaller scope, such as a library or special collection. Thanks for wonderful help from Jeremy York, we recently added hathitrust.org as a collection. This feature is implemented in following way: a request can put an additional parameter "library=hathi" in xOCLCNUM request, the service will only return records which marked as free access in hathitrust.org, when I gather hathitrust data last time, we collected 189,723 free access records with OCLCNUM. Similarly, we have 129,239 free access records from Open Content Alliance, and overall there are 322,629 records about ebooks (mostly free content) in xOCLCNUM service. Besides that, because of the FRBR expansion of xOCLCNUM, when a user requests an OCLCNUM, the service can lookup other OCLCNUM with free access content in same work group, for example, OCLCNUM: 51848364 is a book in copyright, by using the search in a library request, we can tell there are multiple versions of free ebooks about this book, including copies in archive.org and hathitrust.org. I just did a quick calculation, by using the FRBR expansion, 2,446,005 OCLCNUMs can link to a version of ebook. it is still a small percentage of the worldcat database, but I suspect it might be large enough for real world usage, so it would be nice to see this feature get better usage. We have a similar feature in xISBN service, but because most free access books don't have ISBNs, the xOCLCNUM's version might be better for real world usage.

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