No Multi_key sequences for letters with macron below
Latin letters with macron below are used in the standard orthography of various Native American and Indigenous Australian languages, as well as the romanization of various Indian languages.
Letters which have precomposed Unicode characters with macron below are mutually exclusive with letters with macron above.
Existing atomic characters with macron above are as follows:
Āā
/Ǣǣ
/Ēē
/Ḡḡ
/Īī
/Ōō
/Ūū
/Ȳȳ
Existing atomic characters with macron below are as follows:
Ḇḇ
/Ḏḏ
/ẖ
/Ḵḵ
/Ḻḻ
/Ṉṉ
/Ṟṟ
/Ṯṯ
/Ẕẕ
There do exist Ḹḹ
/Ṝṝ
but these also have a dot below which makes them still not overlap as it's no longer just an atomic character + macron. Otherwise, no letters are shared in common.
I would like to suggest perhaps implementing sequences in the format of e.g. <Multi_key> <underscore> <letter>
for letters with macron below as there are no Latin-script letters which would have conflicting sequences for both macron above and below in this format.