I'm thinking of to_md_key(fixed_indexes), to_md_key(some_other_crazy_marker) that map to {md,Key}. Instead of then having sometimes ?FIXED_INDEXES_KEY + another generic path for to_md_key(some_other...). That is, creating a new namespace that can be used for all markers of this kind.  We could also just have atoms I suppose, making this key compatible and other markers in the future being plain atoms, but then someone comes along and adds a marker that's a tuple... I think wrapping it up in {md, <marker key>} would be more future proof.