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#' Crossref TDM authentication
#'
#' @name auth
#' @section Authentication:
#' There's a set of publishers that are involved in the Crossref
#' Text and Data Mining (TDM) program (http://tdmsupport.crossref.org/),
#' which means essentially the publishers
#' deposit URLs for fulltext in Crossref metadata.
#'
#' Authentication is applicable only when the publisher you want to get
#' fulltext from requires it. OA publishers shouldn't need it as you'd
#' expect. There's many publishers that don't share links at all, so they are
#' irrelevant here.
#'
#' For publishers that required authentication, the Crossref TDM
#' program allows for a single token to authenticate across publishers
#' (to make it easier for text miners). The publishers involved with the
#' authentication scheme are really only Elsevier and Wiley.
#'
#' There's a how to guide for Crossref TDM at
#' <http://tdmsupport.crossref.org/researchers/>.
#' Get your Crossref TDM token by registering at
#' <https://apps.crossref.org/clickthrough/researchers>.
#' Save the token in your `.Renviron` file with a new row like
#' `CROSSREF_TDM=your key`. We will read that key in for you - it's best
#' this way rather than passing in a key via a parameter - since you might
#' put that code up on the web somewhere and someone could use your key.
#'
#' @section IP addresses:
#' If you don't know what IP addresses are, check out
#' <https://en.wikipedia.org/wiki/IP_address>. At least Elsevier and
#' I think Wiley also check your IP address in addition to requiring the
#' authentication token. Usually your're good if you're physically at the
#' institution that has access to the publishers content OR on a VPN
#' (i.e., pretending you're there).
#'
#' If you forget about this, you'll get errors about not being authorized.
#' So check and make sure you're on a VPN if you're not physically
#' located at your institution.
#'
#' @section Fences:
#' There's yet another issue to worry about. At least with Elsevier, they
#' have a so-called "fence" - that is, even if an institution has access
#' to Elsevier content, Elsevier doesn't necessarily have the fence
#' turned off - if its not off, you can't get through - if it's off, you can.
#' If you have the right token and you are sure you're on the right
#' IP address, this could be the problem for your lack of access.
#'
#' @section HELP!:
#' If you're having trouble with any of this, get in touch with the
#' package maintainer.
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