diff --git a/Promise.js b/Promise.js
index 6346de7..87b5930 100644
--- a/Promise.js
+++ b/Promise.js
@@ -296,8 +296,19 @@ object.Constructor('IterablePromise', Promise, {
 
 	// XXX BETWEEN...
 	between: function(func){
-		// XXX
-	},
+		var i = 0
+		var j = 0
+		var prev
+		return this.constructor(this,
+			function(e){
+				return i++ > 0 ?
+					[
+						typeof(func) == 'function' ?
+							func.call([prev, e], i, i + j++)
+							: func,
+						e,
+					]
+					: [e] }) },
 
 	// XXX .chain(..) -- see generator.chain(..)
 
diff --git a/generator.js b/generator.js
index 7082e59..dc913c6 100644
--- a/generator.js
+++ b/generator.js
@@ -580,7 +580,7 @@ object.Mixin('AsyncGeneratorProtoMixin', 'soft', {
 		return state },
 
 	// XXX BETWEEN...
-	between: async function(func){
+	between: async function*(func){
 		// XXX
 	},
 
diff --git a/package-lock.json b/package-lock.json
index 1ef2fcf..9b55056 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "ig-types",
-  "version": "6.15.7",
+  "version": "6.17.0",
   "lockfileVersion": 2,
   "requires": true,
   "packages": {
     "": {
       "name": "ig-types",
-      "version": "6.15.7",
+      "version": "6.17.0",
       "license": "BSD-3-Clause",
       "dependencies": {
         "ig-object": "^6.0.0",