fix bug

    expr_mat <- GetAssayData(seurat_obj, slot='counts')

    chunks <- cut(1:nrow(expr_mat), n_chunks)

    expr_mat <- do.call(rbind, lapply(levels(chunks), function(x){

      print(x)

      cur <- expr_mat[chunks == x,]

      cur[cur > 0] <- 1

      cur

    } else{

      # identify genes that are expressed in at least some fraction of cells

      gene_filter <- rowSums(as.matrix(expr_mat)) >= round(fraction*ncol(seurat_obj));

      gene_filter <- rowSums(expr_mat) >= round(fraction*ncol(seurat_obj));

      gene_list <- rownames(seurat_obj)[gene_filter]

    }