diff --git a/spec/range_list_spec.nok b/spec/range_list_spec.cr
similarity index 80%
rename from spec/range_list_spec.nok
rename to spec/range_list_spec.cr
index 29362e0..fc4e356 100644
--- a/spec/range_list_spec.nok
+++ b/spec/range_list_spec.cr
@@ -4,11 +4,11 @@ require "./spec_helper"
 require "../src/timecost/range_list"
 
 describe TimeCost::RangeList do
-	list= TimeCost::RangeList.new
+	list = TimeCost::RangeList.new
 
 	describe ".new" do
 		it "can be created without arguments" do
-			assert_instance_of TimeCost::RangeList, list
+			list.should be_a(TimeCost::RangeList)
 		end
 	end
 
diff --git a/src/timecost/range_list.cr b/src/timecost/range_list.cr
index 4d6171d..6b0673a 100644
--- a/src/timecost/range_list.cr
+++ b/src/timecost/range_list.cr
@@ -4,7 +4,10 @@ module TimeCost
       @ranges = [] of Range
     end
 
-    def add(range : Range)
+    def add(commit : Commit) : RangeList
+    end
+
+    def add(range : Range) : RangeList
       merged = false
       merged_range = nil
 
@@ -34,7 +37,7 @@ module TimeCost
       end
     end
 
-    def sum
+    def sum : Float
       result = 0
       @ranges.each do |r|
         result += r.diff