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#' @details
#'
#' With nine \href{https://www.ncbi.nlm.nih.gov/books/NBK25501/}{Entrez Progamming Utilities},
#' NCBI provides a programmatical interface to the Entrez query and database
#' system for searching and retrieving requested data
#'
#' Each of these tools corresponds to an \code{R} function in the \code{reutils}
#' package described below.
#'
#' The output returned by the EUtils is typically in XML format. To gain access to
#' this output you have several options:
#'
#' \enumerate{
#' \item Use the \code{content(as = "xml")} method to extract the output as an
#' \code{XMLInternalDocument} object and process it further using the
#' facilities provided by the \code{XML} package.
#'
#' \item Use the \code{content(as = "parsed")} method to extract the output
#' into \code{data.frame}s. Note that this is currently only implemented for
#' \emph{docsum}s returned by \code{\link{esummary}}, \emph{uilist}s returned
#' by \code{\link{esearch}}, and the output returned by \code{\link{einfo}}.
#'
#' \item Access specific nodes in the XML tree using XPath expressions with the
#' reference class methods \code{#xmlValue}, \code{#xmlAttr}, or \code{#xmlName}
#' built into \code{\linkS4class{eutil}} objects.
#' }
#'
#' The Entrez Programming Utilities can also generate output in other formats,
#' such as plain-text Fasta or GenBank files for sequence databases,
#' or the MedLine format for the literature database. The type of output is
#' generally controlled by setting the \code{retmode} and \code{rettype} arguments
#' when calling a EUtil.
#' Please check the relevant
#' \href{https://www.ncbi.nlm.nih.gov/books/NBK25497/#chapter2.Usage_Guidelines_and_Requiremen}{usage guidelines}
#' when using these services. Note that Entrez server requests are subject to
#' frequency limits.
#'
#' @section Main functions:
#' \itemize{
#' \item \code{\link{esearch}}: Search and retrieve primary UIDs for use
#' with \code{esummary}, \code{elink}, or \code{efetch}.
#' \code{esearch} additionally returns term translations and optionally
#' stores results for future use in the user's Web Environment.
#'
#' \item \code{\link{esummary}}: Retrieve document summaries from
#' a list of primary UIDs (Provided as a character vector or as an
#' \code{esearch} object).
#'
#' \item \code{\link{egquery}}: Provides Entrez database counts in XML
#' for a single search term using a Global Query.
#'
#' \item \code{\link{einfo}}: Retrieve field names, term counts, last
#' update, and available updates for each database.
#'
#' \item \code{\link{efetch}}: Retrieve data records in a specified
#' format corresponding to a list of primary UIDs or from the user's Web
#' Environment in the Entrez History server.
#'
#' \item \code{\link{elink}}: Returns a list of UIDs (and relevancy
#' scores) from a target database that are related to a list of UIDs in
#' the same database or in another Entrez database.
#'
#' \item \code{\link{epost}}: Uploads primary UIDs to the users's Web
#' Environment on the Entrez history server for subsequent use with
#' \code{esummary}, \code{elink}, or \code{efetch}.
#'
#' \item \code{\link{espell}}: Provide spelling suggestions.
#'
#' \item \code{\link{ecitmatch}}: Retrieves PubMed IDs (PMIDs) that
#' correspond to a set of input citation strings
#'
#' \item \code{\link{content}}: Extract the content of a request from the
#' \code{\linkS4class{eutil}} object returned by any of the above functions.
#' }
#'
#' @section Package options:
#'
#' \pkg{reutils} uses three \code{\link{options}} to configure behaviour:
#'
#' \itemize{
#' \item \code{reutils.email}: NCBI requires that a user of their API provides an
#' email address with a call to Entrez. If you are going to perform a lot
#' of queries consider setting \code{reutils.email} to your email address in
#' your .Rprofile file.
#'
#' \item \code{reutils.show.headlines}: By default \code{\linkS4class{efetch}}
#' objects containing text data show only the first 12 lines. This is quite handy
#' if you have downloaded a fairly large genome in Genbank file format. This
#' can be changed by setting the global option \code{reutils.show.headlines} to
#' another numeric value or \code{NULL}.
#'
#' \item \code{reutils.verbose.queries}: If you perform many queries interactively
#' you might want to get messages announcing the queries you run. You can do so by setting
#' the option \code{reutils.verbose.queries} to \code{TRUE}.
#'
#' \item \code{reutils.test.remote}: Unit tests that require online access to NCBI
#' services are disabled by default, as they cannot be garanteed to be
#' available/working under all circumstances. Set the option
#' code{reutils.test.remote} to \code{TRUE} to run the full suite of tests.
#' }
#'
#' @importFrom methods callNextMethod is new
#' @importFrom stats setNames
#' @example inst/examples/reutils.R
#' @author Gerhard Schöfl \email{gerhard.schofl@gmail.com}
"_PACKAGE"
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