diff --git a/packages/transducers/README.md b/packages/transducers/README.md
index d1f19ed7f1..bd1e20c933 100644
--- a/packages/transducers/README.md
+++ b/packages/transducers/README.md
@@ -605,7 +605,7 @@ reducer and optional initial accumulator/result.
 
 #### `keep<T>(f?: ((x: T) => any)): Transducer<T, T>`
 
-#### `labeled<L, T>(id: L): Transducer<T, [L, T]>`
+#### `labeled<L, T>(id: L | ((x: T) => L)): Transducer<T, [L, T]>`
 
 #### `map<A, B>(fn: (x: A) => B): Transducer<A, B>`
 
@@ -613,7 +613,7 @@ reducer and optional initial accumulator/result.
 
 #### `mapDeep(spec: TransformSpec): Transducer<any, any>`
 
-#### `mapIndexed<A, B>(fn: (i: number, x: A) => B): Transducer<A, B>`
+#### `mapIndexed<A, B>(fn: (i: number, x: A) => B, offset = 0): Transducer<A, B>`
 
 #### `mapKeys(keys: IObjectOf<(x: any) => any>, copy?: boolean): Transducer<any, any>`
 
@@ -641,6 +641,8 @@ reducer and optional initial accumulator/result.
 
 #### `partitionSort<A, B>(n: number, key?: ((x: A) => B), cmp?: Comparator<B>): Transducer<A, A>`
 
+#### `partitionSync<T>(keys: PropertyKey[] | Set<PropertyKey>, keyfn: (x: T) => PropertyKey, reset = true, all = true): Transducer<T, IObjectOf<T>>`
+
 #### `pluck<A, B>(key: PropertyKey): Transducer<A, B>`
 
 #### `rename<A, B>(kmap: IObjectOf<PropertyKey>, rfn?: Reducer<B, [PropertyKey, A]>): Transducer<A[], B>`
diff --git a/packages/transducers/src/index.ts b/packages/transducers/src/index.ts
index e38b33e1c4..13ee5b9186 100644
--- a/packages/transducers/src/index.ts
+++ b/packages/transducers/src/index.ts
@@ -66,6 +66,7 @@ export * from "./xform/page";
 export * from "./xform/partition-by";
 export * from "./xform/partition-of";
 export * from "./xform/partition-sort";
+export * from "./xform/partition-sync";
 export * from "./xform/partition";
 export * from "./xform/pluck";
 export * from "./xform/rename";
diff --git a/packages/transducers/src/xform/partition-sync.ts b/packages/transducers/src/xform/partition-sync.ts
new file mode 100644
index 0000000000..0240cfc092
--- /dev/null
+++ b/packages/transducers/src/xform/partition-sync.ts
@@ -0,0 +1,94 @@
+import { isArray } from "@thi.ng/checks/is-array";
+
+import { IObjectOf } from "@thi.ng/api/api";
+import { Reducer, Transducer } from "../api";
+
+/**
+ * This transducer is intended for synchronization and provenance
+ * tracking of possibly previously merged inputs. It partitions the
+ * input into labeled tuple objects with the object keys obtained from
+ * the user provided `keyfn`. A new result is only produced once values
+ * from **all** given labeled sources have been consumed. Only labels
+ * contained in the provided key set are allowed, others are skipped.
+ * The tuples will contain the most recent consumed value from each
+ * labeled input. In dataflow scenarios this can be used to ensure a
+ * subsequent operation consuming these tuples has all necessary inputs,
+ * regardless of the individual rates of change of each original
+ * (pre-merge) input.
+ *
+ * ```
+ * src = [
+ *   ["a", 1], ["a", 2], ["d", 100], ["b", 10],
+ *   ["b", 11], ["c", 0], ["a", 3]
+ * ];
+ * // form tuples for values only from sources "a" & "b"
+ * // here the label is the first element of each input item
+ * [...iterator(partitionSync(["a", "b"], (x) => x[0]), src)]
+ * // [ { a: ["a", 2], b: ["b", 10] },
+ * //   { b: ["b", 11], a: ["a", 3] } ]
+ * ```
+ *
+ * In addition to the default mode of operation, i.e. waiting for new
+ * values from *all* named inputs before a new tuple is produced, the
+ * behavior for *all but the first tuple* can be changed to emit new
+ * tuples as soon as a new value with a qualifying label has become
+ * available (with other values in the tuple remaining). Compare with
+ * above example:
+ *
+ * ```
+ * // passing `false` to disable tuple reset
+ * [...tx.iterator(tx.partitionSync(["a", "b"], (x) => x[0], false), src)]
+ * // [ { a: ["a", 2], b: ["b", 10] },
+ * //   { a: ["a", 2], b: ["b", 11] },
+ * //   { a: ["a", 3], b: ["b", 11] } ]
+ *
+ * By default, the last emitted tuple is allowed to be incomplete (in
+ * case the input closed). To only allow complete tuples, set the
+ * optional `all` arg to false.
+ *
+ * Note: If the `keys` set of allowed labels is modified externally, the
+ * tuple size will adjust accordingly (only if given as set, will not work
+ * if keys are provided as array).
+ *
+ * @param keys allowed label set
+ * @param keyfn label extraction function
+ * @param reset true if each tuple should contain only new values
+ * @param all true if last tuple is allowed to be incomplete
+ */
+export function partitionSync<T>(keys: PropertyKey[] | Set<PropertyKey>, keyfn: (x: T) => PropertyKey, reset = true, all = true): Transducer<T, IObjectOf<T>> {
+    return ([init, complete, reduce]: Reducer<any, IObjectOf<T>>) => {
+        let curr = {};
+        let first = true;
+        const currKeys = new Set<PropertyKey>();
+        const ks = isArray(keys) ? new Set(keys) : keys;
+        return [
+            init,
+            (acc) => {
+                if ((reset && all && currKeys.size > 0) || (!reset && first)) {
+                    acc = reduce(acc, curr);
+                    curr = undefined;
+                    currKeys.clear();
+                    first = false;
+                }
+                return complete(acc);
+            },
+            (acc, x: T) => {
+                const k = keyfn(x);
+                if (ks.has(k)) {
+                    curr[k] = x;
+                    currKeys.add(k);
+                    if (currKeys.size >= ks.size) {
+                        acc = reduce(acc, curr);
+                        first = false;
+                        if (reset) {
+                            curr = {};
+                            currKeys.clear();
+                        } else {
+                            curr = { ...curr };
+                        }
+                    }
+                }
+                return acc;
+            }];
+    };
+}
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