diff --git a/include/noun/allocate.h b/include/noun/allocate.h
index d2a8a55b66..8533158d99 100644
--- a/include/noun/allocate.h
+++ b/include/noun/allocate.h
@@ -256,6 +256,15 @@
                          ?  u3a_north_is_senior(r, som) \
                          :  u3a_south_is_senior(r, som) )
 
+#     define  u3a_is_mutable(r, som) \
+                ( _(u3a_is_atom(som)) \
+                  ? c3n \
+                  : _(u3a_is_senior(r, som)) \
+                  ? c3n \
+                  : _(u3a_is_junior(r, som)) \
+                  ? c3n \
+                  : (u3a_botox(u3a_to_ptr(som))->use_w == 1) \
+                  ? c3y : c3n )
 
   /**  Globals.
   **/
diff --git a/include/noun/imprison.h b/include/noun/imprison.h
index efda35fab3..f552c87f5e 100644
--- a/include/noun/imprison.h
+++ b/include/noun/imprison.h
@@ -55,6 +55,14 @@
           u3_noun
           u3i_qual(u3_noun a, u3_noun b, u3_noun c, u3_noun d);
 
+        /* u3i_edit():
+        **
+        **   Mutate `big` at axis `axe` with new value `som`
+        **   `axe` is RETAINED.
+        */
+          u3_noun
+          u3i_edit(u3_noun big, u3_noun axe, u3_noun som);
+
         /* u3i_string():
         **
         **   Produce an LSB-first atom from the C string `a`.
diff --git a/noun/imprison.c b/noun/imprison.c
index c08e967f40..c0d16c3dc8 100644
--- a/noun/imprison.c
+++ b/noun/imprison.c
@@ -262,6 +262,150 @@ u3i_qual(u3_noun a, u3_noun b, u3_noun c, u3_noun d)
   return u3i_cell(a, u3i_trel(b, c, d));
 }
 
+static u3_noun
+_edit_cat(u3_noun big, c3_l axe_l, u3_noun som)
+{
+  if ( c3n == u3du(big) ) {
+    return u3m_bail(c3__exit);
+  }
+  else {
+    u3_noun pro;
+    switch ( axe_l ) {
+      case 2:
+        pro = u3nc(som, u3k(u3t(big)));
+        break;
+      case 3:
+        pro = u3nc(u3k(u3h(big)), som);
+        break;
+      default: {
+        c3_l mor_l = u3x_mas(axe_l);
+        pro = ( 2 == u3x_cap(axe_l) )
+            ? u3nc(_edit_cat(u3k(u3h(big)), mor_l, som), u3k(u3t(big)))
+            : u3nc(u3k(u3h(big)), _edit_cat(u3k(u3t(big)), mor_l, som));
+        break;
+      }
+    }
+    u3z(big);
+    return pro;
+  }
+}
+
+static u3_noun
+_edit(u3_noun big, u3_noun axe, u3_noun som)
+{
+  if ( c3y == u3a_is_cat(axe) ) {
+    return _edit_cat(big, (c3_l) axe, som);
+  }
+  else if ( c3n == u3du(big) ) {
+    return u3m_bail(c3__exit);
+  }
+  else {
+    u3_noun mor = u3qc_mas(axe),
+            pro = ( 2 == u3qc_cap(axe) )
+                ? u3nc(_edit(u3k(u3h(big)), mor, som), u3k(u3t(big)))
+                : u3nc(u3k(u3h(big)), _edit(u3k(u3t(big)), mor, som));
+    u3z(mor);
+    u3z(big);
+    return pro;
+  }
+}
+
+static u3_noun _edit_or_mutate_cat(u3_noun, c3_l, u3_noun);
+static u3_noun _edit_or_mutate(u3_noun, u3_noun, u3_noun);
+
+static void
+_mutate_cat(u3_noun big, c3_l axe_l, u3_noun som)
+{
+  if ( c3n == u3du(big) ) {
+    u3m_bail(c3__exit);
+  }
+  else {
+    u3a_cell* cel_u = (void*) u3a_to_ptr(big);
+    switch ( axe_l ) {
+      case 2:
+        u3z(cel_u->hed);
+        cel_u->hed = som;
+        break;
+      case 3:
+        u3z(cel_u->tel);
+        cel_u->tel = som;
+        break;
+      default: {
+        u3_noun* tar = ( 2 == u3x_cap(axe_l) )
+                     ? &(cel_u->hed)
+                     : &(cel_u->tel);
+        *tar = _edit_or_mutate_cat(*tar, u3x_mas(axe_l), som);
+      }
+    }
+  }
+}
+
+static void
+_mutate(u3_noun big, u3_noun axe, u3_noun som)
+{
+  if ( c3y == u3a_is_cat(axe) ) {
+    _mutate_cat(big, (c3_l) axe, som);
+  }
+  else if ( c3n == u3du(big) ) {
+    u3m_bail(c3__exit);
+  }
+  else {
+    u3a_cell* cel_u = (void*) u3a_to_ptr(big);
+    u3_noun mor = u3qc_mas(axe);
+    u3_noun* tar = ( 2 == u3qc_cap(axe) )
+                 ? &(cel_u->hed)
+                 : &(cel_u->tel);
+    *tar = _edit_or_mutate(*tar, mor, som);
+    u3z(mor);
+  }
+}
+
+static u3_noun
+_edit_or_mutate_cat(u3_noun big, c3_l axe_l, u3_noun som)
+{
+  if ( c3y == u3a_is_mutable(u3R, big) ) {
+    _mutate_cat(big, axe_l, som);
+    return big;
+  }
+  else {
+    return _edit_cat(big, axe_l, som);
+  }
+}
+
+static u3_noun
+_edit_or_mutate(u3_noun big, u3_noun axe, u3_noun som)
+{
+  if ( c3y == u3a_is_cat(axe) ) {
+    return _edit_or_mutate_cat(big, (c3_l) axe, som);
+  }
+  else if ( c3y == u3a_is_mutable(u3R, big) ) {
+    _mutate(big, axe, som);
+    return big;
+  }
+  else {
+    return _edit(big, axe, som);
+  }
+}
+
+/* u3i_edit():
+**
+**   Mutate `big` at axis `axe` with new value `som`.
+**   `axe` is RETAINED.
+*/
+u3_noun
+u3i_edit(u3_noun big, u3_noun axe, u3_noun som)
+{
+  switch ( axe ) {
+    case 0:
+      return u3m_bail(c3__exit);
+    case 1:
+      u3z(big);
+      return som;
+    default:
+      return _edit_or_mutate(big, axe, som);
+  }
+}
+
 /* u3i_string():
 **
 **   Produce an LSB-first atom from the C string `a`.
diff --git a/noun/nock.c b/noun/nock.c
index 1ac6de74f9..2150ae223e 100644
--- a/noun/nock.c
+++ b/noun/nock.c
@@ -1591,30 +1591,6 @@ _n_kale(u3_noun a)
   return a;
 }
 
-
-/* _n_edit(): PRODUCE a modified big with its axe axis
- *            changed to som. axe is RETAINED.
- */
-static u3_noun
-_n_edit(u3_noun big, u3_noun axe, u3_noun som)
-{
-  if ( c3y == u3a_is_cat(axe) ) {
-    return u3i_molt(big, axe, som, 0);
-  }
-  else {
-    u3_noun mor = u3qc_mas(axe);
-    if ( c3n == u3du(big) ) {
-      u3m_bail(c3__exit);
-    }
-    u3_noun pro = ( 2 == u3qc_cap(axe) )
-                ? u3nc(_n_edit(u3k(u3h(big)), mor, som), u3k(u3t(big)))
-                : u3nc(u3k(u3h(big)), _n_edit(u3k(u3t(big)), mor, som));
-    u3z(mor);
-    u3z(big);
-    return pro;
-  }
-}
-
 typedef struct {
   u3n_prog* pog_u;
   c3_w     ip_w;
@@ -2325,7 +2301,7 @@ _n_burn(u3n_prog* pog_u, u3_noun bus, c3_ys mov, c3_ys off)
       _n_toss(mov, off);
       top = _n_peek(off);
     edit_in:
-      *top = _n_edit(*top, x, o);
+      *top = u3i_edit(*top, x, o);
       BURN();
   }
 }