Added combination to rank function
Added combination to rank function

thirdwing/mlpack,ajjl/mlpack,trungda/mlpack,ranjan1990/mlpack,darcyliu/mlpack,lezorich/mlpack,palashahuja/mlpack,Azizou/mlpack,theranger/mlpack,erubboli/mlpack,ranjan1990/mlpack,darcyliu/mlpack,BookChan/mlpack,BookChan/mlpack,lezorich/mlpack,stereomatchingkiss/mlpack,thirdwing/mlpack,thirdwing/mlpack,minhpqn/mlpack,datachand/mlpack,bmswgnp/mlpack,chenmoshushi/mlpack,trungda/mlpack,Azizou/mlpack,erubboli/mlpack,palashahuja/mlpack,ersanliqiao/mlpack,stereomatchingkiss/mlpack,datachand/mlpack,ajjl/mlpack,Azizou/mlpack,chenmoshushi/mlpack,erubboli/mlpack,darcyliu/mlpack,minhpqn/mlpack,BookChan/mlpack,ajjl/mlpack,bmswgnp/mlpack,theranger/mlpack,minhpqn/mlpack,ersanliqiao/mlpack,ranjan1990/mlpack,datachand/mlpack,theranger/mlpack,stereomatchingkiss/mlpack,palashahuja/mlpack,chenmoshushi/mlpack,lezorich/mlpack,ersanliqiao/mlpack,bmswgnp/mlpack,trungda/mlpack