diff --git a/xo-arena/include/xo/arena/DArenaHashMap.hpp b/xo-arena/include/xo/arena/DArenaHashMap.hpp
index ceb9961d..df6e15f8 100644
--- a/xo-arena/include/xo/arena/DArenaHashMap.hpp
+++ b/xo-arena/include/xo/arena/DArenaHashMap.hpp
@@ -60,6 +60,41 @@ namespace xo {
         };
 #endif
 
+        /** @class DArenaHashMapUtil
+         *
+         *  @pre
+         *
+         *  control
+         *
+         *   <----------------- control_size(n_slot) ---------------->
+         *   <-stub-> <----------- n_slot ----------> <group> <-stub->
+         *  +--------+-------------------------------+-------+--------+
+         *  | 0xF0   | empty / data / tombstone      | wrap  | 0xF0   |
+         *  +--------+-------------------------------+-------+--------+
+         *             ^                           ^
+         *             |            ...            | control_[stub+i] <--> slots_[i]
+         *  slots      v                           v
+         *           +-------------------------------+
+         *           | {k,v} pairs                   |
+         *           +-------------------------------+
+         *            <--- n_slot key-value pairs -->
+         *
+         *  sizes:
+         *  - stub    before+after bookends.  c_control_stub bytes (16)
+         *  - group   c_group_size. power of 2 (16 bytes)
+         *  - n_slot  hash table slots. power of 2 multiple of c_group_size.
+         *
+         *  control bytes:
+         *  - 0b1xxxxxxx  sentinel bitmask
+         *  - 0xf0        sentinel for before/after stubs (iterator bookends)
+         *  - 0xff        sentinel for empty slot.
+         *  - 0xfe        sentinel for tombstone
+         *  - 0b0xxxxxxx  high bit clear; remainder hold low 7 bits of hash
+         *  - wrap        duplicate first c_group_size bytes (after front stub)
+         *                for SIMD convenience
+         *
+         *  @endpre
+         **/
         struct DArenaHashMapUtil {
             using size_type = std::size_t;
             using control_type = std::uint8_t;
@@ -88,7 +123,7 @@ namespace xo {
 
             /** control: true for sentinel values **/
             static constexpr bool is_sentinel(control_type ctrl) {
-                return ctrl & c_sentinel_mask;
+                return c_sentinel_mask == (ctrl & c_sentinel_mask);
             }
 
             /** control; true for non-sentinel values **/
@@ -254,7 +289,7 @@ namespace xo {
                     /* end stub: iterator bookend */
                     std::fill(this->control_.end() - c_control_stub,
                               this->control_.end(),
-                              c_empty_slot);
+                              c_iterator_bookend);
 
                     this->slots_.resize(n_slot_);
                 }