     let map4 = SimpleExtensionMap(map1, map2)
     assert(map: map4, contains: [ext1, ext2])
 
+    // ext4 has the same message and number as ext2, so should replace it.
     let map5 = SimpleExtensionMap(map1, map2, map3)
     assert(map: map5, contains: [ext1, ext3, ext4])
   }

    map.insert(contentsOf: [ext1, ext2])
    assert(map: map, contains: [ext1, ext2])

    map.insert(contentsOf: [ext3, ext4])
    assert(map: map, contains: [ext1, ext4, ext3])
  }

    let map4 = map1.union(map2)
    assert(map: map4, contains: [ext1, ext2])

    // ext4 has the same message and number as ext2, so should replace it.
    let map5 = map4.union(map3)
    assert(map: map5, contains: [ext1, ext3, ext4])
  }
