assertion failure in update_syntax_cache

[I tried to send this earlier via M-x report-xemacs-bug but it's not appeared, so I thought I'd try again by a different route. Apologies if it turns up twice.] Dear list, I've got a repeatable crash in the latest CVS HEAD of XEmacs (21.5-b6). It occurs when trying to indent a line in C++ mode when a preceding comment block contains an apostrophe (funky, eh?). Place the cursor on the last line of the attached file (apostrophe.cc) and press TAB (for example) and voila: Fatal error: assertion failed, file buffer.h, line 617, ( x) >= ((Charbpos) 1) && x <= ((buf)->text->bufz + 0) I've attached the core backtrace and the lisp backtrace. I'm running this on w2k using a pre-release Cygwin snapshot (but I assume that it's not Cygwin's fault). Cheers, // Conrad