diff --git a/mps/code/config.h b/mps/code/config.h
index 5b73c6d8ff3..021acf277c8 100644
--- a/mps/code/config.h
+++ b/mps/code/config.h
@@ -167,8 +167,9 @@
 /* CONFIG_THREAD_SINGLE -- support single-threaded execution only
  *
  * This symbol causes the MPS to be built for single-threaded
- * execution only, where locks are not needed and so lock operations
- * can be defined as no-ops by lock.h.
+ * execution only, where locks are not needed and so the generic
+ * ("ANSI") lock module lockan.c can be used instead of the
+ * platform-specific lock module.
  */
 
 #if !defined(CONFIG_THREAD_SINGLE)
diff --git a/mps/code/lock.h b/mps/code/lock.h
index 1b421d9eb40..861ac6c272a 100644
--- a/mps/code/lock.h
+++ b/mps/code/lock.h
@@ -128,27 +128,6 @@ extern void LockClaimGlobal(void);
 extern void LockReleaseGlobal(void);
 
 
-#if defined(LOCK)
-/* Nothing to do: functions declared in all lock configurations. */
-#elif defined(LOCK_NONE)
-#define LockSize() MPS_PF_ALIGN
-#define LockInit(lock) UNUSED(lock)
-#define LockFinish(lock) UNUSED(lock)
-#define LockClaimRecursive(lock) UNUSED(lock)
-#define LockReleaseRecursive(lock) UNUSED(lock)
-#define LockClaim(lock) UNUSED(lock)
-#define LockRelease(lock) UNUSED(lock)
-#define LockIsHeld(lock) ((void)lock, FALSE)
-#define LockCheck(lock) ((void)lock, TRUE)
-#define LockClaimGlobalRecursive()
-#define LockReleaseGlobalRecursive()
-#define LockClaimGlobal()
-#define LockReleaseGlobal()
-#else
-#error "No lock configuration."
-#endif  /* LOCK */
-
-
 #endif /* lock_h */
 
 
diff --git a/mps/code/lockan.c b/mps/code/lockan.c
index fe5082a6ebf..8b1b61a5243 100644
--- a/mps/code/lockan.c
+++ b/mps/code/lockan.c
@@ -79,6 +79,12 @@ void (LockReleaseRecursive)(Lock lock)
   --lock->claims;
 }
 
+Bool (LockIsHeld)(Lock lock)
+{
+  AVERT(Lock, lock);
+  return lock->claims > 0;
+}
+
 
 /* Global locking is performed by normal locks.
  * A separate lock structure is used for recursive and
diff --git a/mps/code/lockix.c b/mps/code/lockix.c
index 30fecce488c..4c42d754ba2 100644
--- a/mps/code/lockix.c
+++ b/mps/code/lockix.c
@@ -45,6 +45,7 @@
 
 SRCID(lockix, "$Id$");
 
+#if defined(LOCK)
 
 /* LockStruct -- the MPS lock structure
  *
@@ -260,6 +261,13 @@ void (LockReleaseGlobal)(void)
 }
 
 
+#elif defined(LOCK_NONE)
+#include "lockan.c"
+#else
+#error "No lock configuration."
+#endif
+
+
 /* C. COPYRIGHT AND LICENSE
  *
  * Copyright (C) 2001-2016 Ravenbrook Limited <http://www.ravenbrook.com/>.
diff --git a/mps/code/lockw3.c b/mps/code/lockw3.c
index 09dea1286fb..a471687514e 100644
--- a/mps/code/lockw3.c
+++ b/mps/code/lockw3.c
@@ -31,6 +31,7 @@
 
 SRCID(lockw3, "$Id$");
 
+#if defined(LOCK)
 
 /* .lock.win32: Win32 lock structure; uses CRITICAL_SECTION */
 typedef struct LockStruct {
@@ -165,6 +166,13 @@ void (LockReleaseGlobal)(void)
 }
 
 
+#elif defined(LOCK_NONE)
+#include "lockan.c"
+#else
+#error "No lock configuration."
+#endif
+
+
 /* C. COPYRIGHT AND LICENSE
  *
  * Copyright (C) 2001-2016 Ravenbrook Limited <http://www.ravenbrook.com/>.
diff --git a/mps/design/config.txt b/mps/design/config.txt
index 458ccdfe890..3bf1d96e29c 100644
--- a/mps/design/config.txt
+++ b/mps/design/config.txt
@@ -542,7 +542,8 @@ platform instead.
 
 _`.opt.thread`: ``CONFIG_THREAD_SINGLE`` causes the MPS to be built
 for single-threaded execution only, where locks are not needed and so
-lock operations can be defined as no-ops by ``lock.h``.
+the generic ("ANSI") lock module ``lockan.c`` can be used instead of
+the platform-specific lock module.
 
 _`.opt.poll`: ``CONFIG_POLL_NONE`` causes the MPS to be built without
 support for polling. This means that garbage collections will only
diff --git a/mps/manual/source/topic/arena.rst b/mps/manual/source/topic/arena.rst
index 4fce7a968bb..035f12308ce 100644
--- a/mps/manual/source/topic/arena.rst
+++ b/mps/manual/source/topic/arena.rst
@@ -983,6 +983,13 @@ Arena introspection and debugging
                 mps_arena_postmortem(arena);
             }
 
+    .. warning::
+
+        This function only gives a reliable result in single-threaded
+        programs, and in multi-threaded programs where all threads but
+        one are known to be stopped (as they are when the debugger is
+        decoding the call stack in the use case described above).
+
 
 .. c:function:: mps_bool_t mps_arena_has_addr(mps_arena_t arena, mps_addr_t addr)