A print method for class plist, which are nested lists with a
numeric vector at the lowest level, used as print.default is
rather inefficient (and much uglier) when displaying such nested
lists.
a plist object.
potential further arguments (required by generic), unused by this method
plist objects are created as part of diagnostic output for
TableToLongForm. For more information, refer to the website:
https://www.stat.auckland.ac.nz/~joh024/Research/TableToLongForm/