+The secondary text is of course optional. Not all forms of output distinguish
+between these - sometimes there is just one index. For the concept index, it is
+also possible to set "start" and "end" markers so that the entry lists a range
+of pages. This is how to do that:
+
+ .scindex IID "primary text" "secondary text"
+ <intervening text, should be several pages>
+ .ecindex IID
+
+The IID must be some unique string to tie the entries together.