This is a generic function for which methods can be written: only the
  default method is described here.  Most methods should honor the
  width argument to specify the maximum display width (as measured
  by nchar(type = "width") of the result.
The default method first converts x to character and then
  concatenates the elements separated by ", ".
  If width is supplied and is not NULL, the default method
  returns the first width - 4 characters of the result with
  .... appended, if the full result would use more than
  width characters.