diff --git a/include/xo/object2/DArray.hpp b/include/xo/object2/DArray.hpp
index de65d1d..2dc8e93 100644
--- a/include/xo/object2/DArray.hpp
+++ b/include/xo/object2/DArray.hpp
@@ -6,7 +6,6 @@
 #pragma once
 
 #include <xo/alloc2/GCObject.hpp>
-//#include <xo/alloc2/Collector.hpp>
 #include <xo/alloc2/Allocator.hpp>
 #include <xo/facet/obj.hpp>
 #include <xo/indentlog/print/ppindentinfo.hpp>
@@ -20,7 +19,7 @@ namespace xo {
         namespace detail {
             /** null base case **/
             static inline bool do_array_push_back(DArray *,
-                                                  obj<xo::mm::ACollector>)
+                                                  obj<xo::mm::AAllocator>)
             {
                 return true;
             }
@@ -28,7 +27,7 @@ namespace xo {
             template <typename A, typename... Rest>
             requires (std::convertible_to<A, obj<xo::mm::AGCObject>>)
             static bool do_array_push_back(DArray * lhs,
-                                           obj<xo::mm::ACollector> gc,
+                                           obj<xo::mm::AAllocator> mm,
                                            A arg1,
                                            Rest... rest);
         }
@@ -49,7 +48,7 @@ namespace xo {
             /** type for array size **/
             using size_type = std::uint32_t;
             using AAllocator = xo::mm::AAllocator;
-            using ACollector = xo::mm::ACollector;
+            //using ACollector = xo::mm::ACollector;
             using AGCObject = xo::mm::AGCObject;
             /** gc-centric object visitor **/
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
@@ -114,8 +113,6 @@ namespace xo {
             obj<AGCObject> at(size_type index) const;
 
             const obj<AGCObject> & operator[](size_type index) const noexcept { return elts_[index]; }
-            // TODO: nuke this or provide LValue shim.  need write barrier!
-            obj<AGCObject> & operator[](size_type index) noexcept { return elts_[index]; }
 
             ///@}
             /** @defgroup darray-iterators iterators **/
@@ -128,17 +125,17 @@ namespace xo {
             /** store @p elt at position @p index.
              *  true on success, false otherwise
              **/
-            bool assign_at(obj<ACollector> gc, size_type index, obj<AGCObject> elt) noexcept;
+            bool assign_at(obj<AAllocator> mm, size_type index, obj<AGCObject> elt) noexcept;
 
             /** append @p elt at the end of array.
              *  true on success, false otherwise.
              *  on failure array is unaltered
              **/
-            bool push_back(obj<ACollector> gc, obj<AGCObject> elt) noexcept;
+            bool push_back(obj<AAllocator> mm, obj<AGCObject> elt) noexcept;
 
             template <typename... Args>
             requires (std::convertible_to<Args, obj<AGCObject>> && ...)
-            bool push_back_all(obj<ACollector> gc, Args... args) noexcept;
+            bool push_back_all(obj<AAllocator> mm, Args... args) noexcept;
 
             /** store last element in array into @p elt and decrement array size.
              *  true on success; false on failure (implies array was empty)
@@ -206,10 +203,10 @@ namespace xo {
         DArray *
         DArray::array(obj<AAllocator> mm, Args... args)
         {
-            obj<ACollector> gc = mm.try_to_facet<ACollector>();
+            //obj<ACollector> gc = mm.try_to_facet<ACollector>();
             DArray * result = _empty(mm, sizeof...(args));
             if (result) {
-                detail::do_array_push_back(result, gc, args...);
+                detail::do_array_push_back(result, mm, args...);
             }
             return result;
         }
@@ -218,20 +215,20 @@ namespace xo {
             template <typename A, typename... Rest>
             requires (std::convertible_to<A, obj<xo::mm::AGCObject>>)
             static bool do_array_push_back(DArray * lhs,
-                                           obj<xo::mm::ACollector> gc,
+                                           obj<xo::mm::AAllocator> mm,
                                            A arg1,
                                            Rest... rest)
             {
-                return (lhs->push_back(gc, arg1)
-                        && do_array_push_back(lhs, gc, rest...));
+                return (lhs->push_back(mm, arg1)
+                        && do_array_push_back(lhs, mm, rest...));
             }
         }
 
         template <typename... Args>
         requires (std::convertible_to<Args, obj<xo::mm::AGCObject>> && ...)
         bool
-        DArray::push_back_all(obj<ACollector> gc, Args... args) noexcept {
-            return detail::do_array_push_back(this, gc, args...);
+        DArray::push_back_all(obj<AAllocator> mm, Args... args) noexcept {
+            return detail::do_array_push_back(this, mm, args...);
         }
 
     } /*namespace scm*/
diff --git a/include/xo/object2/DBoolean.hpp b/include/xo/object2/DBoolean.hpp
index 904503c..7be4c07 100644
--- a/include/xo/object2/DBoolean.hpp
+++ b/include/xo/object2/DBoolean.hpp
@@ -5,7 +5,6 @@
 
 #pragma once
 
-//#include <xo/alloc2/Collector.hpp>
 #include <xo/alloc2/Allocator.hpp>
 #include <xo/alloc2/GCObjectVisitor.hpp>
 #include <xo/indentlog/print/ppindentinfo.hpp>
@@ -16,7 +15,6 @@ namespace xo {
     namespace scm {
         struct DBoolean {
             using AAllocator = xo::mm::AAllocator;
-            //using ACollector = xo::mm::ACollector;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
             using AGCObject = xo::mm::AGCObject;
             using VisitReason = xo::mm::VisitReason;
diff --git a/include/xo/object2/DDictionary.hpp b/include/xo/object2/DDictionary.hpp
index 972c71b..3fad8ec 100644
--- a/include/xo/object2/DDictionary.hpp
+++ b/include/xo/object2/DDictionary.hpp
@@ -9,7 +9,6 @@
 #include "DString.hpp"
 #include <xo/alloc2/GCObject.hpp>
 #include <xo/alloc2/GCObjectVisitor.hpp>
-#include <xo/alloc2/Collector.hpp>
 #include <xo/alloc2/Allocator.hpp>
 #include <xo/facet/obj.hpp>
 #include <xo/indentlog/print/ppindentinfo.hpp>
@@ -34,8 +33,6 @@ namespace xo {
             using size_type = std::uint32_t;
             /** xo allocator facet **/
             using AAllocator = xo::mm::AAllocator;
-            /** garbage collector facet **/
-            using ACollector = xo::mm::ACollector;
             /** gc-centric object visitor **/
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
             /** gc-aware object facet **/
@@ -144,12 +141,12 @@ namespace xo {
              *
              *  @return true if key-value pair updated; false if key not found
              **/
-            bool try_update(obj<ACollector> gc, const pair_type & kvpair);
+            bool try_update(obj<AAllocator> mm, const pair_type & kvpair);
 
             /** update key-value pair for existing @p key to map to @p value.
              *  false if @p key not already present.
              **/
-            bool try_update_cstr(obj<ACollector> gc, const char * key, obj<AGCObject> value);
+            bool try_update_cstr(obj<AAllocator> gc, const char * key, obj<AGCObject> value);
 
             /** convenience method:
              *  try_upsert pair (k, @p value), after boxing c-style string @p key with @p mm to get k
@@ -167,7 +164,7 @@ namespace xo {
              *
              *  False if dictionary already at capacity
              **/
-            bool try_upsert(obj<ACollector> gc, const pair_type & kvpair);
+            bool try_upsert(obj<AAllocator> gc, const pair_type & kvpair);
 
             /** upsert key-value pair @p kvpair into dictionary.
              *  If at capacity, expand capacity, getting new memory from @p mm.
@@ -217,7 +214,7 @@ namespace xo {
             /** append {key, value} pair @p kv_pair to this dictionary
              *  Require: @p kv_pair.first not already present in @ref keys_
              **/
-            bool _append_kv_aux(obj<ACollector> gc, const pair_type & kv_pair);
+            bool _append_kv_aux(obj<AAllocator> mm, const pair_type & kv_pair);
             ///@}
 
         private:
diff --git a/include/xo/object2/DInteger.hpp b/include/xo/object2/DInteger.hpp
index ddcd9ba..8bf881a 100644
--- a/include/xo/object2/DInteger.hpp
+++ b/include/xo/object2/DInteger.hpp
@@ -5,7 +5,6 @@
 
 #pragma once
 
-//#include <xo/alloc2/Collector.hpp>
 #include <xo/alloc2/GCObjectVisitor.hpp>
 #include <xo/alloc2/Allocator.hpp>
 #include <xo/indentlog/print/ppindentinfo.hpp>
@@ -16,7 +15,6 @@ namespace xo {
     namespace scm {
         struct DInteger {
             using AAllocator = xo::mm::AAllocator;
-            //using ACollector = xo::mm::ACollector;
             using AGCObjectVisitor = xo::mm::AGCObjectVisitor;
             using AGCObject = xo::mm::AGCObject;
             using VisitReason = xo::mm::VisitReason;
diff --git a/include/xo/object2/DList.hpp b/include/xo/object2/DList.hpp
index a862f18..b5c340c 100644
--- a/include/xo/object2/DList.hpp
+++ b/include/xo/object2/DList.hpp
@@ -64,7 +64,8 @@ namespace xo {
             obj<AGCObject> at(size_type ix) const;
 
             /** assign head **/
-            void assign_head(obj<ACollector> gc, obj<AGCObject> h);
+            void assign_head(obj<AAllocator> mm, obj<AGCObject> h);
+            void assign_head_gc(obj<ACollector> gc, obj<AGCObject> h);
             /** assign rest-pointer. Caller responsible for preserving acyclic property! **/
             void _assign_rest(DList * r);
 
diff --git a/src/object2/DArray.cpp b/src/object2/DArray.cpp
index 1910e8e..44b5a83 100644
--- a/src/object2/DArray.cpp
+++ b/src/object2/DArray.cpp
@@ -75,20 +75,23 @@ namespace xo {
         }
 
         bool
-        DArray::assign_at(obj<ACollector> gc, size_type ix, obj<AGCObject> x) noexcept
+        DArray::assign_at(obj<AAllocator> mm, size_type ix, obj<AGCObject> x) noexcept
         {
             if (ix >= size_)
                 return false;
 
             scope log(XO_DEBUG(true), "need write barrier");
 
-            mm_do_assign(gc, this, &elts_[ix], x);
+            mm.barrier_assign_aux(this,
+                                  elts_[ix].iface(), elts_[ix].opaque_data_addr(),
+                                  x.iface(), x.opaque_data());
+            // mm_do_assign(gc, this, &elts_[ix], x);
 
             return true;
         }
 
         bool
-        DArray::push_back(obj<ACollector> gc, obj<AGCObject> elt) noexcept
+        DArray::push_back(obj<AAllocator> mm, obj<AGCObject> elt) noexcept
         {
             if (size_ >= capacity_) {
                 return false;
@@ -98,7 +101,11 @@ namespace xo {
                 void * mem = &(elts_[size_]);
                 new (mem) obj<AGCObject>();
 
-                mm_do_assign(gc, this, &(elts_[size_]), elt);
+                mm.barrier_assign_aux(this,
+                                      elts_[size_].iface(), elts_[size_].opaque_data_addr(),
+                                      elt.iface(), elt.opaque_data());
+
+                //mm_do_assign(gc, this, &(elts_[size_]), elt);
 
                 ++(this->size_);
 
diff --git a/src/object2/DDictionary.cpp b/src/object2/DDictionary.cpp
index 43a4e1c..2f52cae 100644
--- a/src/object2/DDictionary.cpp
+++ b/src/object2/DDictionary.cpp
@@ -108,7 +108,7 @@ namespace xo {
         }
 
         bool
-        DDictionary::try_update(obj<ACollector> gc, const pair_type & kv_pair)
+        DDictionary::try_update(obj<AAllocator> mm, const pair_type & kv_pair)
         {
             for (size_type i = 0, n = keys_->size(); i < n; ++i) {
                 auto key_i = obj<AGCObject,DString>::from((*keys_)[i]);
@@ -116,7 +116,7 @@ namespace xo {
                 assert(key_i);
 
                 if (*(key_i.data()) == *(kv_pair.first)) {
-                    values_->assign_at(gc, i, kv_pair.second);
+                    values_->assign_at(mm, i, kv_pair.second);
                     return true;
                 }
             }
@@ -125,7 +125,7 @@ namespace xo {
         }
 
         bool
-        DDictionary::try_update_cstr(obj<ACollector> gc, const char * key, obj<AGCObject> value)
+        DDictionary::try_update_cstr(obj<AAllocator> mm, const char * key, obj<AGCObject> value)
         {
             for (size_type i = 0, n = keys_->size(); i < n; ++i) {
                 auto key_i = obj<AGCObject,DString>::from((*keys_)[i]);
@@ -133,7 +133,7 @@ namespace xo {
                 assert(key_i);
 
                 if (strcmp(key, key_i->data()) == 0) {
-                    values_->assign_at(gc, i, value);
+                    values_->assign_at(mm, i, value);
 
                     return true;
                 }
@@ -148,8 +148,8 @@ namespace xo {
             const DString * k1 = DString::from_cstr(mm, key_cstr);
 
             if (k1) {
-                obj<ACollector> gc = mm.try_to_facet<ACollector>();
-                return this->try_upsert(gc, std::make_pair(k1, value));
+                //obj<ACollector> gc = mm.try_to_facet<ACollector>();
+                return this->try_upsert(mm, std::make_pair(k1, value));
             }
 
             return false;
@@ -167,23 +167,23 @@ namespace xo {
         }
 
         bool
-        DDictionary::try_upsert(obj<ACollector> gc, const pair_type & kv_pair)
+        DDictionary::try_upsert(obj<AAllocator> mm, const pair_type & kv_pair)
         {
-            if (this->try_update(gc, kv_pair))
+            if (this->try_update(mm, kv_pair))
                 return true;
 
             if (keys_->size() == keys_->capacity())
                 return false;
 
-            return this->_append_kv_aux(gc, kv_pair);
+            return this->_append_kv_aux(mm, kv_pair);
         }
 
         bool
         DDictionary::upsert(obj<AAllocator> mm, const pair_type & kv_pair)
         {
-            obj<ACollector> gc = mm.try_to_facet<ACollector>();
+            //obj<ACollector> gc = mm.try_to_facet<ACollector>();
 
-            if (this->try_update(gc, kv_pair))
+            if (this->try_update(mm, kv_pair))
                 return true;
 
             // key not present -> must expand {key array, value array}
@@ -204,19 +204,19 @@ namespace xo {
                 }
             }
 
-            return this->_append_kv_aux(gc, kv_pair);
+            return this->_append_kv_aux(mm, kv_pair);
         }
 
         bool
-        DDictionary::_append_kv_aux(obj<ACollector> gc, const pair_type & kv_pair)
+        DDictionary::_append_kv_aux(obj<AAllocator> mm, const pair_type & kv_pair)
         {
             DString * key = const_cast<DString *>(kv_pair.first);
 
             bool ok
-                = keys_->push_back(gc, obj<AGCObject,DString>(key));
+                = keys_->push_back(mm, obj<AGCObject,DString>(key));
 
             if (ok) {
-                ok = values_->push_back(gc, kv_pair.second);
+                ok = values_->push_back(mm, kv_pair.second);
 
                 if (!ok) {
                     // since we couldn't insert value, also drop key
diff --git a/src/object2/DList.cpp b/src/object2/DList.cpp
index 7208bb9..0b694aa 100644
--- a/src/object2/DList.cpp
+++ b/src/object2/DList.cpp
@@ -120,7 +120,20 @@ namespace xo {
         }
 
         void
-        DList::assign_head(obj<ACollector> gc, obj<AGCObject> rhs)
+        DList::assign_head(obj<AAllocator> mm, obj<AGCObject> rhs)
+        {
+            scope log(XO_DEBUG(true), xtag("mm.data", mm.data_));
+
+            mm.barrier_assign_aux(this,
+                                  head_.iface(), head_.opaque_data_addr(),
+                                  rhs.iface(), rhs.opaque_data());
+
+            //mm_do_assign(gc, this, &head_, rhs);
+        }
+
+        // vestigial. used in MockCollector
+        void
+        DList::assign_head_gc(obj<ACollector> gc, obj<AGCObject> rhs)
         {
             scope log(XO_DEBUG(true), xtag("gc.data", gc.data_));
 
diff --git a/utest/DArray.test.cpp b/utest/DArray.test.cpp
index af26c71..8935360 100644
--- a/utest/DArray.test.cpp
+++ b/utest/DArray.test.cpp
@@ -30,12 +30,12 @@ namespace xo {
             REQUIRE(arr.size() == 0);
             REQUIRE(arr.capacity() == 0);
 
-            REQUIRE(arr.is_empty());;
+            REQUIRE(arr.is_empty());
 
-            // null_gc: for no memory barrier
-            obj<ACollector> null_gc;
+            // null_mm: for no memory barrier
+            obj<AAllocator> null_mm;
 
-            REQUIRE(arr.push_back(null_gc, ListOps::nil()) == false);
+            REQUIRE(arr.push_back(null_mm, ListOps::nil()) == false);
         }
 
         TEST_CASE("DArray-empty", "[object2][DArray]")
@@ -61,7 +61,7 @@ namespace xo {
                               .size_ = 4*1024 };
             DArena arena = DArena::map(cfg);
             auto alloc = with_facet<AAllocator>::mkobj(&arena);
-            obj<ACollector> null_gc;
+            obj<AAllocator> null_mm;
 
             DArray * arr = DArray::_empty(alloc, 16);
             REQUIRE(arr != nullptr);
@@ -70,7 +70,7 @@ namespace xo {
 
             obj<AGCObject> elt = DInteger::box<AGCObject>(alloc, 42);
 
-            bool ok = arr->push_back(null_gc, elt);
+            bool ok = arr->push_back(null_mm, elt);
 
             REQUIRE(ok == true);
             REQUIRE(arr->is_empty() == false);
@@ -84,7 +84,7 @@ namespace xo {
                               .size_ = 4*1024 };
             DArena arena = DArena::map(cfg);
             auto alloc = with_facet<AAllocator>::mkobj(&arena);
-            obj<ACollector> null_gc;
+            obj<AAllocator> null_mm;
 
             DArray * arr = DArray::_empty(alloc, 4);
             REQUIRE(arr != nullptr);
@@ -96,7 +96,7 @@ namespace xo {
                 REQUIRE(arr->size() == i);
 
                 obj<AGCObject> elt = DInteger::box<AGCObject>(alloc, 100 + i);
-                bool ok = arr->push_back(null_gc, elt);
+                bool ok = arr->push_back(null_mm, elt);
                 REQUIRE(ok == true);
 
                 REQUIRE(arr->capacity() == 4);
@@ -113,7 +113,7 @@ namespace xo {
             auto alloc = with_facet<AAllocator>::mkobj(&arena);
 
             DArray * arr = DArray::_empty(alloc, 2);
-            obj<ACollector> null_gc;
+            obj<AAllocator> null_mm;
 
             REQUIRE(arr != nullptr);
             REQUIRE(arr->capacity() == 2);
@@ -123,11 +123,11 @@ namespace xo {
             obj<AGCObject> e2 = DInteger::box<AGCObject>(alloc, 2);
             obj<AGCObject> e3 = DInteger::box<AGCObject>(alloc, 3);
 
-            REQUIRE(arr->push_back(null_gc, e1) == true);
+            REQUIRE(arr->push_back(null_mm, e1) == true);
             REQUIRE(arr->size() == 1);
-            REQUIRE(arr->push_back(null_gc, e2) == true);
+            REQUIRE(arr->push_back(null_mm, e2) == true);
             REQUIRE(arr->size() == 2);
-            REQUIRE(arr->push_back(null_gc, e3) == false);
+            REQUIRE(arr->push_back(null_mm, e3) == false);
             REQUIRE(arr->size() == 2);
             REQUIRE(arr->capacity() == 2);
         }
@@ -140,7 +140,7 @@ namespace xo {
             auto alloc = with_facet<AAllocator>::mkobj(&arena);
 
             DArray * arr = DArray::_empty(alloc, 4);
-            obj<ACollector> null_gc;
+            obj<AAllocator> null_mm;
 
             REQUIRE(arr != nullptr);
             REQUIRE(arr->size() == 0);
@@ -150,9 +150,9 @@ namespace xo {
             obj<AGCObject> e1 = DInteger::box<AGCObject>(alloc, 200);
             obj<AGCObject> e2 = DInteger::box<AGCObject>(alloc, 300);
 
-            arr->push_back(null_gc, e0);
-            arr->push_back(null_gc, e1);
-            arr->push_back(null_gc, e2);
+            arr->push_back(null_mm, e0);
+            arr->push_back(null_mm, e1);
+            arr->push_back(null_mm, e2);
 
             REQUIRE(arr->size() == 3);