rcrossref interacts with the main Crossref metadata search API at
https://github.com/CrossRef/rest-api-doc/blob/master/rest_api.md, the old metadata
search API at http://search.labs.crossref.org/, their DOI Content Negotiation
service at http://www.crosscite.org/cn/, and the Text and Data Mining
project http://tdmsupport.crossref.org/.Note that functions that use the OpenURL service are now deprecated, and will be defunct
soon.