The function downloads US stock-market data provided
  by Robert Shiller which he used in his book
  ‘Irrational Exhuberance’.  Since the data are
  provided in Excel format, package readxl is
  required.
The downloaded Excel gets a date prefix (today in
  format YYYYMMDD) and is stored in directory
  dest.dir. Before any download is attempted,
  the function checks whether a file with today's
  prefix exist in dest.dir; if yes, the file is
  used.