[plt-scheme] What is the default object writer?

I have a class that implements printable<%>, but I only want to
customize display, not write.  What should I do in the custom-write
method?  How do I call the default object writer, i.e., the code that
prints unreadable values, as described in section 12.7.13?
http://docs.plt-scheme.org/reference/printing.html#%28part._print-unreadable%29

Also, perhaps unrelated, but what exactly does the print-unreadable
parameter do?  Section 12.5 says it is "A parameter that controls
printing values that have no readable form", but it doesn't actually
say what the difference is between #t and #f.  It links to section
12.7, but that does not mention this parameter at all.

--dougorleans at gmail.com