"TextDocument": such packages then provide S3 text document
  classes extending the virtual base class (such as the
  AnnotatedPlainTextDocument objects provided by package
    All extension classes must provide an as.character()
  method which extracts the natural language text in documents of the
  respective classes in a content() and meta()
  methods for accessing the (possibly raw) document content and metadata.
  In addition, the infrastructure features the generic functions
  words(), sents(), etc., for which
  extension classes can provide methods giving a structured view of the
  text contained in documents of these classes (returning, e.g., a
  character vector with the word tokens in these documents, and a list
  of such character vectors).
AnnotatedPlainTextDocument,
  CoNLLTextDocument,
  TaggedTextDocument, and
  WordListDocument
  for the text document classes provided by package