Not only the grouping sets alone, the chaining of them.

E.g.

   GROUP BY tenant
          , GROUPING SETS ( (year, month)
                          , (year)
                          )

is equivalant to

   GROUP BY GROUPING SETS ( (tenant, year, month)
                          , (tenant, year)
                          )

and

   GROUP BY tenant, year
          , GROUPING SETS ( (month)
                          , ()
                          )

(however, I find the last one odd: I like to keep the "tenant" part seperate from the actual grupings you'd like to do).