diff --git a/src/db/db/dbAsIfFlatRegion.cc b/src/db/db/dbAsIfFlatRegion.cc
index 03782957f..0bfc03df1 100644
--- a/src/db/db/dbAsIfFlatRegion.cc
+++ b/src/db/db/dbAsIfFlatRegion.cc
@@ -440,6 +440,14 @@ template void AsIfFlatRegion::produce_markers_for_grid_check<db::UnitTrans> (con
 EdgePairs
 AsIfFlatRegion::grid_check (db::Coord gx, db::Coord gy) const
 {
+  if (gx < 0 || gy < 0) {
+    throw tl::Exception (tl::to_string (tr ("Grid check requires a positive grid value")));
+  }
+
+  if (gx == 0 && gy == 0) {
+    return EdgePairs ();
+  }
+
   std::auto_ptr<db::FlatEdgePairs> res (new db::FlatEdgePairs ());
 
   for (RegionIterator p (begin_merged ()); ! p.at_end (); ++p) {
@@ -567,6 +575,14 @@ AsIfFlatRegion::snapped_polygon (const db::Polygon &poly, db::Coord gx, db::Coor
 RegionDelegate *
 AsIfFlatRegion::snapped (db::Coord gx, db::Coord gy)
 {
+  if (gx < 0 || gy < 0) {
+    throw tl::Exception (tl::to_string (tr ("Grid check requires a positive grid value")));
+  }
+
+  if (gx == 0 && gy == 0) {
+    return this;
+  }
+
   std::auto_ptr<FlatRegion> new_region (new FlatRegion (merged_semantics ()));
 
   gx = std::max (db::Coord (1), gx);
diff --git a/src/db/db/dbDeepRegion.cc b/src/db/db/dbDeepRegion.cc
index e4b6c82da..dbef515b5 100644
--- a/src/db/db/dbDeepRegion.cc
+++ b/src/db/db/dbDeepRegion.cc
@@ -716,7 +716,7 @@ DeepRegion::to_string (size_t nmax) const
 EdgePairs
 DeepRegion::grid_check (db::Coord gx, db::Coord gy) const
 {
-  if (gx <= 0 || gy <= 0) {
+  if (gx < 0 || gy < 0) {
     throw tl::Exception (tl::to_string (tr ("Grid check requires a positive grid value")));
   }
 
@@ -725,6 +725,10 @@ DeepRegion::grid_check (db::Coord gx, db::Coord gy) const
     return db::AsIfFlatRegion::grid_check (gx, gy);
   }
 
+  if (gx == 0) {
+    return EdgePairs ();
+  }
+
   ensure_merged_polygons_valid ();
 
   db::Layout &layout = m_merged_polygons.layout ();
@@ -796,7 +800,7 @@ DeepRegion::angle_check (double min, double max, bool inverse) const
 RegionDelegate *
 DeepRegion::snapped (db::Coord gx, db::Coord gy)
 {
-  if (gx <= 0 || gy <= 0) {
+  if (gx < 0 || gy < 0) {
     throw tl::Exception (tl::to_string (tr ("Snapping requires a positive grid value")));
   }
 
@@ -805,6 +809,10 @@ DeepRegion::snapped (db::Coord gx, db::Coord gy)
     return db::AsIfFlatRegion::snapped (gx, gy);
   }
 
+  if (! gx) {
+    return this;
+  }
+
   ensure_merged_polygons_valid ();
 
   db::cell_variants_collector<db::GridReducer> vars (gx);
diff --git a/src/drc/unit_tests/drcSimpleTests.cc b/src/drc/unit_tests/drcSimpleTests.cc
index b3d225ce8..197d7b4df 100644
--- a/src/drc/unit_tests/drcSimpleTests.cc
+++ b/src/drc/unit_tests/drcSimpleTests.cc
@@ -102,7 +102,7 @@ TEST(2)
   db::compare_layouts (_this, layout, au, db::NoNormalization);
 }
 
-TEST(3)
+TEST(3_Flat)
 {
   std::string rs = tl::testsrc ();
   rs += "/testdata/drc/drcSimpleTests_3.drc";
@@ -141,3 +141,43 @@ TEST(3)
 
   db::compare_layouts (_this, layout, au, db::NoNormalization);
 }
+
+TEST(4_Hierarchical)
+{
+  std::string rs = tl::testsrc ();
+  rs += "/testdata/drc/drcSimpleTests_4.drc";
+
+  std::string input = tl::testsrc ();
+  input += "/testdata/drc/drctest.gds";
+
+  std::string au = tl::testsrc ();
+  au += "/testdata/drc/drcSimpleTests_au4.oas";
+
+  std::string output = this->tmp_file ("tmp.gds");
+
+  {
+    //  Set some variables
+    lym::Macro config;
+    config.set_text (tl::sprintf (
+        "$drc_test_source = '%s'\n"
+        "$drc_test_target = '%s'\n"
+      , input, output)
+    );
+    config.set_interpreter (lym::Macro::Ruby);
+    EXPECT_EQ (config.run (), 0);
+  }
+
+  lym::Macro drc;
+  drc.load_from (rs);
+  EXPECT_EQ (drc.run (), 0);
+
+  db::Layout layout;
+
+  {
+    tl::InputStream stream (output);
+    db::Reader reader (stream);
+    reader.read (layout);
+  }
+
+  db::compare_layouts (_this, layout, au, db::NoNormalization);
+}
diff --git a/src/drc/unit_tests/drcSuiteTests.cc b/src/drc/unit_tests/drcSuiteTests.cc
index 94e043626..b134a853d 100644
--- a/src/drc/unit_tests/drcSuiteTests.cc
+++ b/src/drc/unit_tests/drcSuiteTests.cc
@@ -72,25 +72,36 @@ void runtest (tl::TestBase *_this, int mode)
   db::compare_layouts (_this, layout, au, db::WriteOAS);
 }
 
-TEST(1)
+TEST(1_Flat)
 {
   runtest (_this, 1);
 }
 
-TEST(2)
+TEST(2_BigFlat)
 {
   test_is_long_runner ();
   runtest (_this, 2);
 }
 
-TEST(3)
+TEST(3_Tiled)
 {
   test_is_long_runner ();
   runtest (_this, 3);
 }
 
-TEST(4)
+TEST(4_BigTiled)
 {
   test_is_long_runner ();
   runtest (_this, 4);
 }
+
+TEST(5_Hier)
+{
+  runtest (_this, 5);
+}
+
+TEST(6_BigHier)
+{
+  test_is_long_runner ();
+  runtest (_this, 6);
+}
diff --git a/testdata/drc/drcSimpleTests_4.drc b/testdata/drc/drcSimpleTests_4.drc
new file mode 100644
index 000000000..579156b06
--- /dev/null
+++ b/testdata/drc/drcSimpleTests_4.drc
@@ -0,0 +1,49 @@
+
+# Foreign cell test
+
+source($drc_test_source, "TOPTOP_SMALL")
+target($drc_test_target)
+
+cell("TOPTOP_SMALL")
+
+l1_flat = input(1)
+l1_flat.is_deep? && raise("l1_flat should not be deep")
+
+is_deep? && raise("is_deep? is true")
+
+deep
+
+is_deep? || raise("is_deep? is false")
+
+l1 = input(1)
+l1.is_deep? || raise("l1 should be deep")
+l2 = input(2)
+l2.is_deep? || raise("l2 should be deep")
+
+flat
+
+is_deep? && raise("is_deep? is true")
+
+l2_flat = input(2)
+l2_flat.is_deep? && raise("l2_flat should not be deep")
+
+l1.output(1, 0)
+l2.output(2, 0)
+
+l1_flat.output(11, 0)
+l2_flat.output(12, 0)
+
+l1.and(l2).output(1000, 0)
+l1_flat.and(l2).output(1001, 0)
+l1.and(l2_flat).output(1002, 0)
+l1_flat.and(l2_flat).output(1003, 0)
+
+l1.space(0.2).output(1010, 0)
+l1_flat.space(0.2).output(1011, 0)
+
+l1.separation(l2, 0.3).output(1020, 0)
+l1_flat.separation(l2, 0.3).output(1021, 0)
+l1.separation(l2_flat, 0.3).output(1022, 0)
+l1_flat.separation(l2_flat, 0.3).output(1023, 0)
+
+
diff --git a/testdata/drc/drcSimpleTests_au4.oas b/testdata/drc/drcSimpleTests_au4.oas
new file mode 100644
index 000000000..aa37d1e7d
Binary files /dev/null and b/testdata/drc/drcSimpleTests_au4.oas differ
diff --git a/testdata/drc/drcSuiteTests.drc b/testdata/drc/drcSuiteTests.drc
index 8490b026e..88765993e 100644
--- a/testdata/drc/drcSuiteTests.drc
+++ b/testdata/drc/drcSuiteTests.drc
@@ -4,12 +4,16 @@ def message(m)
   $stdout.flush
 end
 
-def run_testsuite(dm, ic, tiled = false)
+def expect_eq(a, b)
+  a == b || raise("unexpected value #{a.to_s}, should be #{b.to_s}")
+end
+
+def run_testsuite(dm, ic, tiled = false, hier = false)
 
   has_float_range = ((0..0.5).max != 0)
 
   lb = 100
-  
+
   a = input(RBA::LayerInfo::new(1, 0))
   b = input(2)
   c = input(3)
@@ -21,16 +25,16 @@ def run_testsuite(dm, ic, tiled = false)
   layers.each { |l| h[l] = true }
   h[RBA::LayerInfo::new(1, 0)] || raise("missing layer 1/0 in layers list")
   
-  c.is_merged? == false || raise("unexpected value")
+  expect_eq(c.is_merged?, false)
   
   message "--- general #{lb}"
   
   l1 = a&b
   l1.output(lb, dm)
-  l1.is_empty? && raise("must not be empty")
+  expect_eq(l1.is_empty?, false)
   
   a.and(b).xor(l1).is_empty? || raise("xor not empty")
-  tiled || (a.and(b).is_merged? == true || raise("unexpected value"))
+  tiled || hier || expect_eq(a.and(b).is_merged?, true)
   
   a.xor(b).output(RBA::LayerInfo::new(lb + 1, dm))
   a.xor(b).xor(a ^ b).is_empty? || raise("xor not empty")
@@ -43,28 +47,36 @@ def run_testsuite(dm, ic, tiled = false)
   a.join(b).xor(a + b).is_empty? || raise("xor not empty")
 
   if !tiled
-    a.join(b).data.size == 16 * ic || raise("unexpected shape count")
+    expect_eq(a.join(b).data.size, 16 * ic)
   end
   
-  c.raw.is_clean? == false || raise("unexpected value")
-  # c.raw switched the semantics
-  c.is_clean? == false || raise("unexpected value")
-  (c.area / ic).to_s == "10.0" || raise("unexpected value")
-  (c.edges.length / ic).to_s == "18.0" || raise("unexpected value")
-  (c.perimeter / ic).to_s == "18.0" || raise("unexpected value")
-  c.clean
-  c.is_clean? == true || raise("unexpected value")
-  (c.area / ic).to_s == "9.0" || raise("unexpected value")
-  (c.edges.length / ic).to_s == "14.0" || raise("unexpected value")
-  (c.perimeter / ic).to_s == "14.0" || raise("unexpected value")
-  (c.dup.area / ic).to_s == "9.0" || raise("unexpected value")
-  (c.raw.clean.area / ic).to_s == "9.0" || raise("unexpected value")
-  (c.area / ic).to_s == "9.0" || raise("unexpected value")
-  (c.raw.area / ic).to_s == "10.0" || raise("unexpected value")
-  c.clean
+  # NOTE: there is no clean/raw semantics in deep mode
+  if ! hier
+    expect_eq(c.raw.is_clean?, false)
+    # c.raw switched the semantics
+    expect_eq(c.is_clean?, false)
+    expect_eq((c.area / ic).to_s, "10.0")
+    expect_eq((c.edges.length / ic).to_s, "18.0")
+    expect_eq((c.perimeter / ic).to_s, "18.0")
+    c.clean
+    expect_eq(c.is_clean?, true)
+  end
+
+  expect_eq((c.area / ic).to_s, "9.0")
+  expect_eq((c.edges.length / ic).to_s, "14.0")
+  expect_eq((c.perimeter / ic).to_s, "14.0")
+  expect_eq((c.dup.area / ic).to_s, "9.0")
+
+  # NOTE: there is no clean/raw semantics in deep mode
+  if ! hier
+    expect_eq((c.raw.clean.area / ic).to_s, "9.0")
+    expect_eq((c.area / ic).to_s, "9.0")
+    expect_eq((c.raw.area / ic).to_s, "10.0")
+    c.clean
+  end
 
   if ic == 1    
-    c.bbox.to_s == "(-4,-5;0,-2)" || raise("unexpected value")
+    expect_eq(c.bbox.to_s, "(-4,-5;0,-2)")
   end
   
   lb += 10 #110
@@ -80,15 +92,15 @@ def run_testsuite(dm, ic, tiled = false)
   a.edges.end_segments(0, 0.5).extended_out(0.01).output(lb + 7, dm)
   a.edges.end_segments(0.5, 0.5).extended_out(0.01).output(lb + 8, dm)
   
-  a.polygons? == true || raise("unexpected value")
-  a.edges? == false || raise("unexpected value")
-  a.edge_pairs? == false || raise("unexpected value")
-  a.edges.edge_pairs? == false || raise("unexpected value")
-  a.edges.polygons? == false || raise("unexpected value")
-  a.edges.edges? == true || raise("unexpected value")
-  a.space(0.5).edge_pairs? == true || raise("unexpected value")
-  a.space(0.5).polygons? == false || raise("unexpected value")
-  a.space(0.5).edges? == false || raise("unexpected value")
+  expect_eq(a.polygons?, true)
+  expect_eq(a.edges?, false)
+  expect_eq(a.edge_pairs?, false)
+  expect_eq(a.edges.edge_pairs?, false)
+  expect_eq(a.edges.polygons?, false)
+  expect_eq(a.edges.edges?, true)
+  expect_eq(a.space(0.5).edge_pairs?, true)
+  expect_eq(a.space(0.5).polygons?, false)
+  expect_eq(a.space(0.5).edges?, false)
   
   lb += 10 #120
   message "--- extended #{lb}"
@@ -132,7 +144,7 @@ def run_testsuite(dm, ic, tiled = false)
   b.interacting(a).in(b).output(lb + 1, dm)
   (b|a).not_in(b).output(lb + 2, dm)
   x.in(b).output(lb + 3, dm)
-  b.sized(0.1).in(b).is_empty? == true || raise("unexpected value")
+  expect_eq(b.sized(0.1).in(b).is_empty?, true)
   
   lb += 10 #170
   message "--- inside, outside, overlapping, interacting #{lb}"
@@ -209,14 +221,18 @@ def run_testsuite(dm, ic, tiled = false)
   lb += 10 #180
   message "--- merge #{lb}"
   
-  c.raw
+  if !hier
+    c.raw
+  end
   if !tiled
-    c.merged.is_merged? == true || raise("unexpected value")
+    expect_eq(c.merged.is_merged?, true)
   end
   c.merged.output(lb, dm)
   c.merged(2).output(lb + 1, dm)
   c.merged(3).output(lb + 2, dm)
-  c.clean
+  if !hier
+    c.clean
+  end
   cdup = c.dup
   cdup.merge.xor(c.merged).output(lb + 3, dm)
   cdup.xor(c.merged).output(lb + 4, dm)
@@ -252,9 +268,11 @@ def run_testsuite(dm, ic, tiled = false)
   a.non_rectangles.output(lb + 1, dm)
   a.rectilinear.output(lb + 2, dm)
   a.non_rectilinear.output(lb + 3, dm)
-  c.raw
-  c.non_rectangles.output(lb + 4, dm)
-  c.clean
+  if !hier
+    c.raw
+    c.non_rectangles.output(lb + 4, dm)
+    c.clean
+  end
   c.rectangles.output(lb + 5, dm)
   
   lb += 10 #210
@@ -561,12 +579,18 @@ if $drc_test_mode == 1
 
   source($drc_test_source, "TOP")
   target($drc_test_target, "TOP")
+
+  flat
+
   run_testsuite(0, 1)
 
 elsif $drc_test_mode == 2
 
   target($drc_test_target, "TOPTOP")
   source($drc_test_source, "TOPTOP")
+
+  flat
+
   run_testsuite(0, 900)
 
 elsif $drc_test_mode == 3
@@ -592,5 +616,23 @@ elsif $drc_test_mode == 4
 
   run_testsuite(0, 900, true)
 
+elsif $drc_test_mode == 5
+
+  source($drc_test_source, "TOP")
+  target($drc_test_target, "TOP")
+
+  deep
+
+  run_testsuite(0, 1, false, true)
+
+elsif $drc_test_mode == 6
+
+  target($drc_test_target, "TOPTOP")
+  source($drc_test_source, "TOPTOP")
+
+  deep
+
+  run_testsuite(0, 900, false, true)
+
 end
 
diff --git a/testdata/drc/drcSuiteTests_au5.oas b/testdata/drc/drcSuiteTests_au5.oas
new file mode 100644
index 000000000..c49c2c616
Binary files /dev/null and b/testdata/drc/drcSuiteTests_au5.oas differ
diff --git a/testdata/drc/drcSuiteTests_au6.oas b/testdata/drc/drcSuiteTests_au6.oas
new file mode 100644
index 000000000..bf5fabfd9
Binary files /dev/null and b/testdata/drc/drcSuiteTests_au6.oas differ
diff --git a/testdata/drc/drctest.gds b/testdata/drc/drctest.gds
index daa662289..f1eb87e11 100644
Binary files a/testdata/drc/drctest.gds and b/testdata/drc/drctest.gds differ