create an include expression

lib/flipper/expression/builder.rb

@@ -53,6 +53,10 @@ def less_than_or_equal_to(object)
       alias lte less_than_or_equal_to
       alias less_than_or_equal less_than_or_equal_to
 
+      def include(object)
+        build({ Include: [self, object] })
+      end
+
       def percentage_of_actors(object)
         build({ PercentageOfActors: [self, build(object)] })
       end

lib/flipper/expressions/include.rb

@@ -0,0 +1,9 @@
+module Flipper
+  module Expressions
+    class Include < Comparable
+      def self.operator
+        :include?
+      end
+    end
+  end
+end

spec/flipper/expression_spec.rb

@@ -79,6 +79,19 @@
       ])
     end
 
+    it "can build Include" do
+      expression = described_class.build({
+        "Inlcude" => [["hello"], "hello"]
+      })
+
+      expect(expression).to be_instance_of(Flipper::Expression)
+      expect(expression.function).to be(Flipper::Expressions::Include)
+      expect(expression.args).to eq([
+        Flipper.constant(["hello"]),
+        Flipper.constant("hello"),
+      ])
+    end
+
     it "can build NotEqual" do
       expression = described_class.build({
         "NotEqual" => [