[texhax] output routine question: index "continued" lines

[Stan]

I'm trying to set up a basic output routine that will handle automatic  
insertion of "continued" lines at the top of pages where appropriate.  I  
wanted to see what the macros on pages 262-263 of the Texbook do, but I  
got an error ("Forbidden control sequence found while scanning use of  
\next") when I tried to use them with a simple example, replacing  
"\normaloutput" with "\plainoutput".  Has anyone else gotten these macros  
to work?

More to the point, how can I modify an output routine (say, \plainoutput)  
to tack on one or two extra lines at the top of a page when necessary, and  
then remove the same number of lines from the bottom and put them back at  
the top of the list of things that did not make it onto the current page?

All of my attempts have resulted in the same type of error that results,  
for example, from changing

	\output={\plainoutput}

to

	\output={\plainoutput!}

In other words, pages were ejected until some limitation was hit.  (In my  
case, I got a "TeX capacity exceeded, sorry [main memory size=2500000]"  
message.)

Thanks for any help.