+Note the cheesy \tr{.o.hi} rule: It records the dependency of the
+interface (\tr{.hi}) file on the source. The rule says a \tr{.hi}
+file can be made from a \tr{.o} file by doing... nothing. Which is
+true.
+
+(Sophisticated \tr{make} variants may achieve some of the above more
+elegantly. What we've shown should work with any \tr{make}.)
+