-contain at most one character with strictly positive width,
-which must be the first,
-and contain no characters of negative width.
+contain at most one spacing character,
+which must be the first.
+.IP
+Up to \fBCCHARW_MAX\fP\-1 nonspacing characters may follow.
+Additional nonspacing characters are ignored.
+.IP
+The string may contain a single control character instead.
+In that case, no nonspacing characters are allowed.