aggregate_function KEEP 
( DENSE_RANK FIRST ORDER BY
  expr [ DESC | ASC ] [ NULLS { FIRST | LAST }]
  [, expr [ DESC | ASC ] [ NULLS { FIRST | LAST }]]...
)
[OVER query_partitioning_clause]

For information on the symbols and conventions of Backus-Naur Form (BNF) syntax, see Appendix A.