+Emacs supports two comment styles simultaneously in any one syntax
+table. This is for the sake of C++. Each style of comment syntax has
+its own comment-start sequence and its own comment-end sequence. Each
+comment must stick to one style or the other; thus, if it starts with
+the comment-start sequence of style ``b'', it must also end with the
+comment-end sequence of style ``b''.
+
+@c #### Compatibility note; index here.
+As an extension to GNU Emacs 19 and 20, XEmacs supports two arbitrary
+comment-start sequences and two arbitrary comment-end sequences. (Thus
+the need for 8 flags.) GNU Emacs restricts the comment-start sequences
+to start with the same character, XEmacs does not. This means that for
+two-character sequences, where GNU Emacs uses the @samp{b} flag, XEmacs
+uses the digit flags @samp{5}--@samp{8}.
+
+A one character comment-end sequence applies to the ``b'' style if its
+first character has the @samp{b} flag set; otherwise, it applies to the
+``a'' style. The @samp{a} flag is optional. These flags have no effect
+on non-comment characters; two-character styles are determined by the
+digit flags.
+