Add Cycles::Within#notation_extended_by(count)

lib/sof/cycles/within.rb

@@ -12,6 +12,13 @@ def self.recurring? = false
 
       def to_s = "#{volume}x within #{date_range}"
 
+      def notation_extended_by(count)
+        Parser.load(
+          parser.to_h.merge(period_count: period_count + count)
+        ).to_s
+      end
+      alias_method :grant_extension, :notation_extended_by
+
       def date_range
         return humanized_span unless active?
 

spec/sof/cycles/within_spec.rb

@@ -34,6 +34,17 @@ module SOF
       given: "_", returns: ("2020-08-01".to_date + 180.days)
     it_behaves_like "last_completed is", :too_late
 
+    describe "#notation_extended_by" do
+      it "increases the period by the given amount" do
+        aggregate_failures do
+          expect(cycle.notation).to eq("V2W180DF2020-08-01")
+
+          notation = cycle.notation_extended_by(30)
+          expect(notation).to eq("V2W210DF2020-08-01")
+        end
+      end
+    end
+
     describe "#recurring?" do
       it "does not repeat" do
         expect(cycle).not_to be_recurring