Very low use, only two distinct font files, Apple Chancery.ttf and Hoefler Text.ttc
have it so it really doesn't worth the size addition and so, but one may argue that
whole ligature caret is low use but guess we better to encourage GDEF one anyway.
AAT::Lookup has no other way to detect whether it is returned from
a real and sanitized font data or from a null pool, this checks if
the table has been recognized valid by sanitizer by checking
table's major version which is zero if returned from a null pool and
non-zero if is from a sanitized font data, it is expected the other
calls of the table (unlikely to have more calls however) also do a
similar version check before calling the lookups used on the table.
Ebrahim Byagowi