UAX #29 definesSpacingMark as:

Grapheme_Cluster_Break ≠ Extend,and
General_Category = Spacing_Mark,or
any of the following (which have General_Category = Other_Letter):
U+0E33 ( ำ ) THAI CHARACTER SARA AM
U+0EB3 ( ຳ ) LAO VOWEL SIGN AM

Exceptions: The following (which have General_Category = Spacing_Mark and would otherwise be included) are specifically excluded:
[24 exception characters]

Inthis crate's implementation of rule GB9a, only the "General_Category = Spacing_Mark" part is checked. This crate doesn't check that Grapheme_Cluster_Break ≠ Extend or implement any of the 24 exclusions or 2 inclusions. The impact of this is very minor though, since it only affects a small set of characters, and only in extended mode.

(originally noted in#107)