diff -r 944f02992369 -r 84de0ba791af src/cx/map.h
--- a/src/cx/map.h	Wed Nov 05 22:39:39 2025 +0100
+++ b/src/cx/map.h	Wed Nov 05 23:04:46 2025 +0100
@@ -583,6 +583,26 @@
 CX_EXPORT int cxMapListIntersection(CxMap *dst, const CxMap *src, const CxList *keys,
         cx_clone_func clone_func, const CxAllocator *clone_allocator, void *data);
 
+/**
+ * Clones entries into a map if their key does not exist yet.
+ *
+ * If you want to calculate the union of two maps into a fresh new map,
+ * you can proceed as follows:
+ * 1. Clone the first map into a fresh, empty map.
+ * 2. Use this function to clone the second map into the result from step 1.
+ *
+ * @param dst the destination map
+ * @param src the map to clone the entries from
+ * @param clone_func the clone function for the values
+ * @param clone_allocator the allocator that is passed to the clone function
+ * @param data optional additional data that is passed to the clone function
+ * @retval zero when the elements were successfully cloned
+ * @retval non-zero when an allocation error occurred
+ */
+cx_attr_nonnull_arg(1, 2, 3)
+CX_EXPORT int cxMapUnion(CxMap *dst, const CxMap *src,
+        cx_clone_func clone_func, const CxAllocator *clone_allocator, void *data);
+
 #ifdef    __cplusplus
 } // extern "C"
 #endif