diff --git a/filtering.go b/filtering.go
index 7eacd49..82db975 100644
--- a/filtering.go
+++ b/filtering.go
@@ -14,12 +14,12 @@ func (f Filter) Get(key string) (values []string, ok bool) {
 	return values, ok
 }
 
-func (f Filter) Size() int {
+func (f Filter) Count() int {
 	return len(f)
 }
 
-func (f Filter) IsNotEmpty() bool {
-	return len(f) > 0
+func (f Filter) Add(key, val string) {
+	f[key] = append(f[key], val)
 }
 
 func (f Filter) ValueAt(val string, index int) string {
@@ -84,7 +84,7 @@ func MakeFilterString(prefix string, filters Filter, validFilters []string) (res
 		prefix += "."
 	}
 
-	if !filters.IsNotEmpty() {
+	if filters.Count() == 0 {
 		return "", true
 	}