[Tagging] Can we assume surface=cobblestone:flattened to be an exact duplicate of surface=sett?

[Mateusz Konieczny]

So far I assumed that surface=cobblestone:flattened is an exact duplicate of
surface=sett but looking at

https://lists.openstreetmap.org/pipermail/tagging/2018-January/thread.html#34931
https://wiki.openstreetmap.org/wiki/Template:Map_Features:surface
https://wiki.openstreetmap.org/w/index.php?title=Tag:surface%3Dcobblestone:flattened&action=edit&redlink=1

it seems to not be actually true.

I am asking as
- I want to document also this surface value
- I am considering treating it as a missing data in StreetComplete
(and ask user to provide surface info, like for missing surface value)
- Right now in the upcoming surface edit mode it is treated as surface=sett
synonym - maybe it should be changed and be treated as missing surface info

So how it should be treated?
Like surface=cobblestone where it can be surface=sett or 
surface=unhewn_cobblestone so ideally it would be retagged with more specific values? 
( https://wiki.openstreetmap.org/wiki/Tag:surface%3Dcobblestone )
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