--- doc/whatis.texi.orig	2014-09-13 16:40:50.000000000 +0900
+++ doc/whatis.texi	2014-09-13 16:41:45.000000000 +0900
@@ -35,10 +35,11 @@
 @end macro
 @end ifinfo
 
-@tex
-@def@xsub#1{$_{#1}$}
-@global@let@sub=@xsub
-@end tex
+@iftex
+@macro sub{arg}
+@textsubscript{\arg\}
+@end macro
+@end iftex
 
 @ifhtml
 @macro sub{arg}
