diff --git a/src/db/db/dbDeepRegion.cc b/src/db/db/dbDeepRegion.cc
index 0d7d725de..bed425c3d 100644
--- a/src/db/db/dbDeepRegion.cc
+++ b/src/db/db/dbDeepRegion.cc
@@ -962,10 +962,31 @@ DeepRegion::xor_with (const Region &other, db::PropertyConstraint property_const
 
     //  Implement XOR as (A-B)+(B-A) - only this implementation
     //  is compatible with the local processor scheme
-    DeepLayer n1 (and_or_not_with (other_deep, false, property_constraint));
-    DeepLayer n2 (other_deep->and_or_not_with (this, false, property_constraint));
 
+    //  Prepare a version of "other_deep" that is mapped into the hierarchy space
+    //  of "this"
+    std::unique_ptr<DeepRegion> other_deep_mapped;
+    if (&other_deep->deep_layer ().layout () == &deep_layer ().layout ()) {
+      //  shallow copy for reconfiguration (progress etc.)
+      other_deep_mapped.reset (new DeepRegion (other_deep->deep_layer ()));
+    } else {
+      //  deep copy with mapped hierarchy
+      other_deep_mapped.reset (new DeepRegion (deep_layer ().derived ()));
+      other_deep_mapped->deep_layer ().add_from (other_deep->deep_layer ());
+    }
+
+    other_deep_mapped->set_strict_handling (strict_handling ());
+    other_deep_mapped->set_base_verbosity (base_verbosity ());
+    if (report_progress ()) {
+      other_deep_mapped->enable_progress (progress_desc () + tl::to_string (tr (" - reverse part")));
+    } else {
+      other_deep_mapped->disable_progress ();
+    }
+
+    DeepLayer n1 (and_or_not_with (other_deep_mapped.get (), false, property_constraint));
+    DeepLayer n2 (other_deep_mapped->and_or_not_with (this, false, property_constraint));
     n1.add_from (n2);
+
     return new DeepRegion (n1);
 
   }
diff --git a/src/db/db/dbHierProcessor.cc b/src/db/db/dbHierProcessor.cc
index cf8bbfcb5..9987451e2 100644
--- a/src/db/db/dbHierProcessor.cc
+++ b/src/db/db/dbHierProcessor.cc
@@ -1239,7 +1239,8 @@ private:
   void
   collect_instance_interactions (const db::CellInstArray *inst1, const db::CellInstArray *inst2)
   {
-    //  TODO: this algorithm is not in particular effective for identical arrays
+    //  TODO: this algorithm is not in particular effective for identical arrays or for arrays
+    //  vs. single instances
 
     const db::Cell &cell1 = mp_subject_layout->cell (inst1->object ().cell_index ());
     const db::Cell &cell2 = mp_intruder_layout->cell (inst2->object ().cell_index ());
diff --git a/src/db/unit_tests/dbHierProcessorTests.cc b/src/db/unit_tests/dbHierProcessorTests.cc
index 7e9fe165f..504f36c2e 100644
--- a/src/db/unit_tests/dbHierProcessorTests.cc
+++ b/src/db/unit_tests/dbHierProcessorTests.cc
@@ -1278,3 +1278,9 @@ TEST(FlatOperation)
   run_test_bool22_flat (_this, "hlp17_flat.oas", TMAndNot, 100, 101);
 }
 
+TEST(Arrays)
+{
+  //  Large arrays, NOT
+  run_test_bool2 (_this, "hlp18.oas", TMNot, 100);
+}
+
diff --git a/testdata/algo/hlp18.oas b/testdata/algo/hlp18.oas
new file mode 100644
index 000000000..2c45f1226
Binary files /dev/null and b/testdata/algo/hlp18.oas differ