There are numerous occasions in page layout when it is necessary to store some text for a period of time without actually printing it. Footnotes are the most obvious example: the text of the footnote usually appears in the input well before the place on the page where it is to be printed is reached. In fact, the place where it is output normally depends on how big it is, which implies that there must be a way to process the footnote at least enough to decide its size without printing it.

troff provides a mechanism called a diversion for doing this processing. Any part of the output may be diverted into a macro instead of being printed, and then at some convenient time the macro may be put back into the input.

The command .di xy begins a diversion _ all subsequent output is collected into the macro xy until the command .di with no arguments is encountered. This terminates the diversion. The processed text is available at any time thereafter, simply by giving the command

1 ^xy

2 The vertical size of the last finished diversion is contained in the built-in number register dn .

As a simple example, suppose we want to implement a `keep-release' operation, so that text between the commands .KS and .KE will not be split across a page boundary (as for a figure or table). Clearly, when a .KS is encountered, we have to begin diverting the output so we can find out how big it is. Then when a .KE is seen, we decide whether the diverted text will fit on the current page, and print it either there if it fits, or at the top of the next page if it doesn't. So:

1 2 ^de KS \e" start keep ^br \e" start fresh line ^ev 1 \e" collect in new environment ^fi \e" make it filled text ^di XX \e" collect in XX ^^

2

1 2 ^de KE \e" end keep ^br \e" get last partial line ^di \e" end diversion ^if \e\en(dn>=\e\en(.t .bp \e" bp if doesn't fit ^nf \e" bring it back in no-fill ^XX \e" text ^ev \e" return to normal environment ^^

2 Recall that number register nl is the current position on the output page. Since output was being diverted, this remains at its value when the diversion started. dn is the amount of text in the diversion; .t (another built-in register) is the distance to the next trap, which we assume is at the bottom margin of the page. If the diversion is large enough to go past the trap, the .if is satisfied, and a .bp is issued. In either case, the diverted output is then brought back with .XX . It is essential to bring it back in no-fill mode so troff will do no further processing on it.

This is not the most general keep-release, nor is it robust in the face of all conceivable inputs, but it would require more space than we have here to write it in full generality. This section is not intended to teach everything about diversions, but to sketch out enough that you can read existing macro packages with some comprehension.