% pandoc -t native -s -M title=New
% Old
^D
Pandoc (Meta {unMeta = fromList [("title",MetaString "New")]})
[]

% pandoc -t native -s -M foo=1 -M foo=2
^D
Pandoc (Meta {unMeta = fromList [("foo",MetaList [MetaString "1",MetaString "2"])]})
[]