Client for various 'CrossRef' 'APIs', including 'metadata' search with their old and newer search 'APIs', get 'citations' in various formats (including 'bibtex', 'citeproc-json', 'rdf-xml', etc.), convert 'DOIs' to 'PMIDs', and 'vice versa', get citations for 'DOIs', and get links to full text of articles when available.