// Display the nationalities with at least female artists,
// in descending order by nationality
db.Artists.aggregate( [
   { $match : { Gender : "Female" }},
   { $sortByCount : "$Nationality" },
   { $project : { _id : 0, nationality : "$_id", count : 1}},
   { $match : { count : { $gte : 50 }}}
] ) ;