Suppose you want a different navigation bar for some of your HTML output files, depending on the name of the chapter from which the files are generated. One way would be to use a conditional expression (see §37.6.4.2 Using conditional expressions) to check the current chapter file name and choose the code for the navigation bar. For example:
[NavBar]
; Configure navigation bar for roadmap:
<$_if ($$_currbase is "user_roadmap")> <$rmap>
; Configure navigation bar for Programmer's Guide topics:
<$_elseif ($$_currbase is "bgp_user")> <$pgnav>
<$_elseif ($$_currbase is "mld_user")> <$pgnav>
... (long list of similar clauses)
<$_elseif ($$_currbase is "pga_user")> <$pgnav>
; Configure navigation bar for function topics, by default:
<$_else>
<p>
<a href="functions.htm">Function Index</a>
</p>
<$_endif>
Instead, you could use a list indexed by the value of $$_currbase, with each list value a macro call (or HTML code):
[navmap] 0 = <p><a href="functions.htm">Function Index</a></p> user_roadmap = <$rmap> bgp_user = <$pgnav> mld_user = <$pgnav> ... pga_user = <$pgnav>
[NavBar] <$navmap[$$_currbase]>
The 0 (zero) list item corresponds to the <$_else> clause in the original [NavBar] macro, and is used if the specified index (the value of $$_currbase) is not found. Instead of a macro call the value of this list item is straight HTML code, which works as long as the code is all on one line. You could just as well use a macro call for the zero value, like the rest of the list items.