(defun mm-charset-after (&optional pos)
"Return charset of a character in current buffer at position POS.
-If POS is nil, it defauls to the current point.
+If POS is nil, it defaults to the current point.
If POS is out of range, the value is nil.
If the charset is `composition', return the actual one."
(let ((char (char-after pos)) charset)