Extract PubMed ID from elink object

Extract PubMed ID from an elink object.

Usage

# S3 method for class 'elink'
extract_pmid(x, linkname = NULL, quietly = FALSE, no_result = "error", ...)

x: An elink object, generated by rentrez::entrez_link and it's derivatives (e.g. citedby_pmid).
linkname: linkname as specified for Entrez API; only necessary if more than one linkname is returned from PubMed.
quietly: Suppress PubMed linkname message when linkname is not specified and multiple results exist in an elink object.
no_result: The type of condition that should be signaled when no PubMed results exist in a response; one of "error", "warning" (default), "message" or "none".
...: Unused, included for generic consistency only.