diff --git a/include/noun/allocate.h b/include/noun/allocate.h
index 23fd55891..11199014d 100644
--- a/include/noun/allocate.h
+++ b/include/noun/allocate.h
@@ -250,6 +250,15 @@
                          ?  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 efda35fab..f552c87f5 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/jets/f/hike.c b/jets/f/hike.c
index 567b1ca0d..6b1bb5b36 100644
--- a/jets/f/hike.c
+++ b/jets/f/hike.c
@@ -89,6 +89,7 @@
   u3qf_hike(u3_noun axe,
             u3_noun pac)
   {
+    c3_assert(0);
     if ( (u3_nul == pac) ) {
       return u3nc(0, u3k(axe));
     }
diff --git a/noun/imprison.c b/noun/imprison.c
index 09126c14f..daf59a1d3 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 459f6cb2e..b1af247b7 100644
--- a/noun/nock.c
+++ b/noun/nock.c
@@ -526,6 +526,22 @@ _n_nock_on(u3_noun bus, u3_noun fol)
 #define SLIB 70
 #define SLIS 71
 #define SAVE 72
+// nock 12
+#define MUTH 73
+#define KUTH 74
+#define MUTT 75
+#define KUTT 76
+#define MUSM 77
+#define KUSM 78
+#define MUTB 79
+#define MUTS 80
+#define MITB 81
+#define MITS 82
+#define KUTB 83
+#define KUTS 84
+#define KITB 85
+#define KITS 86
+#define LAST 87
 
 /* _n_arg(): return the size (in bytes) of an opcode's argument
  */
@@ -538,6 +554,7 @@ _n_arg(c3_y cod_y)
     case SAMB: case SANB: case SBIP: case SBIN:
     case SLIB: case SKIB: case KICB: case TICB:
     case BUSH: case BAST: case BALT:
+    case MUTB: case KUTB: case MITB: case KITB:
       return sizeof(c3_y);
 
     case FASK: case FASL: case FISL: case FISK:
@@ -545,13 +562,14 @@ _n_arg(c3_y cod_y)
     case SAMS: case SANS: case SIPS: case SINS:
     case SLIS: case SKIS: case KICS: case TICS:
     case SUSH: case SAST: case SALT: 
+    case MUTS: case KUTS: case MITS: case KITS:
       return sizeof(c3_s);
 
     case SWIP: case SWIN:
       return sizeof(c3_l);
 
     default:
-      c3_assert( cod_y <= SAVE );
+      c3_assert( cod_y < LAST );
       return 0;
   }
 }
@@ -657,6 +675,7 @@ _n_melt(u3_noun ops, c3_w* byc_w, c3_w* cal_w,
         
         case BUSH: case FIBK: case FIBL:
         case SANB: case LIBL: case LIBK:
+        case KITB: case MITB:
           a_w = (*lit_w)++;
           if ( a_w <= 0xFF ) {
             siz_y[i_w] = 2;
@@ -854,6 +873,7 @@ _n_prog_asm(u3_noun ops, u3n_prog* pog_u, u3_noun sip)
         /* 8-bit direct args */
         case FABK: case FABL:
         case LITB: case LILB: 
+        case MUTB: case KUTB:
         case SAMB: 
           buf_y[i_w--] = (c3_y) u3t(op);
           buf_y[i_w]   = (c3_y) cod;
@@ -862,6 +882,7 @@ _n_prog_asm(u3_noun ops, u3n_prog* pog_u, u3_noun sip)
         /* 16-bit direct args */
         case FASK: case FASL:
         case LILS: case LITS:
+        case MUTS: case KUTS:
         case SAMS: case SIPS: case SINS: {
           c3_s off_s   = u3t(op);
           buf_y[i_w--] = (c3_y) (off_s >> 8);
@@ -885,6 +906,7 @@ _n_prog_asm(u3_noun ops, u3n_prog* pog_u, u3_noun sip)
         case FIBK: case FIBL: 
         case LIBK: case LIBL:
         case BUSH: case SANB:
+        case KITB: case MITB:
           _n_prog_asm_inx(buf_y, &i_w, lit_s, cod);
           pog_u->lit_u.non[lit_s++] = u3k(u3t(op));
           break;
@@ -992,6 +1014,10 @@ static char* opcode_names[] = {
   "balt", "salt",
   "skib", "skis", "slib", "slis",
   "save",
+  "muth", "kuth", "mutt", "kutt",
+  "musm", "kusm",
+  "mutb", "muts", "mitb", "mits",
+  "kutb", "kuts", "kitb", "kits",
 };
 #endif
 
@@ -1289,6 +1315,38 @@ _n_comp(u3_noun* ops, u3_noun fol, c3_o los_o, c3_o tel_o)
       ++tot_w; _n_emit(ops, (c3y == los_o) ? WILS : WISH);
       break;
 
+    case 12: {
+      u3_noun axe, nef;
+      u3x_cell(arg, &hed, &tel);
+      u3x_cell(hed, &axe, &nef);
+      tot_w += _n_comp(ops, tel, c3n, c3n);
+      ++tot_w; _n_emit(ops, SWAP);
+      tot_w += _n_comp(ops, nef, c3n, c3n);
+
+      ++tot_w;
+      switch ( axe ) {
+        case 2:
+          _n_emit(ops, (c3y == los_o) ? MUTH : KUTH);
+          break;
+
+        case 3:
+          _n_emit(ops, (c3y == los_o) ? MUTT : KUTT);
+          break;
+
+        case u3x_sam:
+          _n_emit(ops, (c3y == los_o) ? MUSM : KUSM);
+          break;
+
+        default:
+          op_y = (c3y == los_o)
+               ? (axe <= 0xFF) ? MUTB : (axe <= 0xFFFF) ? MUTS : MITB  // overflows to MITS
+               : (axe <= 0xFF) ? KUTB : (axe <= 0xFFFF) ? KUTS : KITB; // overflows to KITS
+          _n_emit(ops, u3nc(op_y, u3k(axe)));
+          break;
+      }
+      break;
+    }
+
     default:
       u3m_bail(c3__exit);
       return 0;
@@ -1573,6 +1631,10 @@ _n_burn(u3n_prog* pog_u, u3_noun bus, c3_ys mov, c3_ys off)
     &&do_balt, &&do_salt,
     &&do_skib, &&do_skis, &&do_slib, &&do_slis,
     &&do_save,
+    &&do_muth, &&do_kuth, &&do_mutt, &&do_kutt,
+    &&do_musm, &&do_kusm,
+    &&do_mutb, &&do_muts, &&do_mitb, &&do_mits,
+    &&do_kutb, &&do_kuts, &&do_kitb, &&do_kits,
   };
 
   u3j_site* sit_u;
@@ -2158,6 +2220,89 @@ _n_burn(u3n_prog* pog_u, u3_noun bus, c3_ys mov, c3_ys off)
       *top = x;
       u3z(o);
       BURN();
+
+    do_kuth:
+      x    = _n_pep(mov, off);
+      top  = _n_swap(mov, off);
+      goto muth_in;
+    do_muth:
+      x    = _n_pep(mov, off);
+      _n_toss(mov, off);
+      top  = _n_peek(off);
+    muth_in:
+      o    = *top;
+      *top = u3nc(x, u3k(u3t(o)));
+      u3z(o);
+      BURN();
+
+    do_kutt:
+      x    = _n_pep(mov, off);
+      top  = _n_swap(mov, off);
+      goto mutt_in;
+    do_mutt:
+      x    = _n_pep(mov, off);
+      _n_toss(mov, off);
+      top  = _n_peek(off);
+    mutt_in:
+      o    = *top;
+      *top = u3nc(u3k(u3h(o)), x);
+      u3z(o);
+      BURN();
+
+    do_kusm:
+      x    = _n_pep(mov, off);
+      top  = _n_swap(mov, off);
+      goto musm_in;
+    do_musm:
+      x    = _n_pep(mov, off);
+      _n_toss(mov, off);
+      top  = _n_peek(off);
+    musm_in:
+      o    = *top;
+      *top = u3nt(u3k(u3h(o)), x, u3k(u3t(u3t(o))));
+      u3z(o);
+      BURN();
+
+    do_kitb:
+      x = pog_u->lit_u.non[pog[ip_w++]];
+      goto kut_in;
+
+    do_kits:
+      x = pog_u->lit_u.non[_n_resh(pog, &ip_w)];
+      goto kut_in;
+
+    do_kuts:
+      x = _n_resh(pog, &ip_w);
+      goto kut_in;
+
+    do_kutb:
+      x = pog[ip_w++];
+    kut_in:
+      o   = _n_pep(mov, off);
+      top = _n_swap(mov, off);
+      goto edit_in;
+
+    do_mitb:
+      x = pog_u->lit_u.non[pog[ip_w++]];
+      goto mut_in;
+
+    do_mits:
+      x = pog_u->lit_u.non[_n_resh(pog, &ip_w)];
+      goto mut_in;
+
+    do_muts:
+      x = _n_resh(pog, &ip_w);
+      goto mut_in;
+
+    do_mutb:
+      x = pog[ip_w++];
+    mut_in:
+      o = _n_pep(mov, off);
+      _n_toss(mov, off);
+      top = _n_peek(off);
+    edit_in:
+      *top = u3i_edit(*top, x, o);
+      BURN();
   }
 }