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.