-immediately. The end of an indented section - the OUT token - is
-ignored precisely when the indent was ignored. To keep track of this we
-need a small stack of flags, which is easily stored as bits in an
-`unsigned long`. This will never overflow and the scanner only allows
-20 levels of indentation.
+immediately or nothing can be shifted (in which case we reduce, and try
+again). The end of an indented section - the OUT token - is ignored
+precisely when the indent was ignored. To keep track of this we need a
+small stack of flags, which is easily stored as bits in an `unsigned
+long`. This will never overflow and the scanner only allows 20 levels
+of indentation.