diff --git a/README.md b/README.md
index 2560eb2..4bf25c0 100644
--- a/README.md
+++ b/README.md
@@ -127,6 +127,12 @@ See LICENSE file.
 
 ## History
 
+Jun 13 2012 v0.13
+  - Added maxlevel option.
+  - Fixed key sorting such that `true` and `'true'` are always sorted in
+    the same order (for a more stable output).
+  - Removed addresses from names of temporary variables (for stable output).
+
 Jun 12 2012 v0.12
   - Added options to configure serialization process.
   - Added 'goto' to the list of keywords for Lua 5.2.
diff --git a/src/serpent.lua b/src/serpent.lua
index 627b785..6785ca3 100644
--- a/src/serpent.lua
+++ b/src/serpent.lua
@@ -1,4 +1,4 @@
-local n, v = "serpent", 0.12 -- (C) 2012 Paul Kulchenko; MIT License
+local n, v = "serpent", 0.13 -- (C) 2012 Paul Kulchenko; MIT License
 local c, d = "Paul Kulchenko", "Serializer and pretty printer of Lua data types"
 local snum = {[tostring(1/0)]='1/0 --[[math.huge]]',[tostring(-1/0)]='-1/0 --[[-math.huge]]',[tostring(0/0)]='0/0'}
 local badtype = {thread = true, userdata = true}
@@ -14,8 +14,9 @@ local function s(t, opts)
   local name, indent, fatal = opts.name, opts.indent, opts.fatal
   local sparse, custom, huge = opts.sparse, opts.custom, not opts.nohuge
   local space, maxl = (opts.compact and '' or ' '), (opts.maxlevel or math.huge)
-  local seen, sref = {}, {}
-  local function gensym(val) return tostring(val):gsub("[^%w]","") end
+  local seen, sref, syms, symn = {}, {}, {}, 0
+  local function gensym(val) return tostring(val):gsub("[^%w]",""):gsub("(%d%w+)",
+    function(s) if not syms[s] then symn = symn+1; syms[s] = symn end return syms[s] end) end
   local function safestr(s) return type(s) == "number" and (huge and snum[tostring(s)] or s)
     or type(s) ~= "string" and tostring(s) -- escape NEWLINE/010 and EOF/026
     or ("%q"):format(s):gsub("\010","n"):gsub("\026","\\026") end
@@ -26,14 +27,13 @@ local function s(t, opts)
     local n = name == nil and '' or name
     local plain = type(n) == "string" and n:match("^[%l%u_][%w_]*$") and not keyword[n]
     local safe = plain and n or '['..safestr(n)..']'
-    return (path or '')..(plain and path and '.' or '')..safe, safe
-  end
+    return (path or '')..(plain and path and '.' or '')..safe, safe end
   local function alphanumsort(o, n)
     local maxn = tonumber(n) or 12
     local function padnum(d) return ("%0"..maxn.."d"):format(d) end
     table.sort(o, function(a,b)
-      return tostring(a):gsub("%d+",padnum) < tostring(b):gsub("%d+",padnum) end)
-  end
+      return tostring(a):gsub("%d+",padnum)..type(a)
+           < tostring(b):gsub("%d+",padnum)..type(b) end) end
   local function val2str(t, name, indent, path, plainindex, level)
     local ttype, level = type(t), (level or 0)
     local spath, sname = safename(path, name)
diff --git a/t/test.lua b/t/test.lua
index ac8752f..72d8f99 100644
--- a/t/test.lua
+++ b/t/test.lua
@@ -52,3 +52,12 @@ assert(_a.list[4] == 'f', "specific table element preserves its value: failed")
 _a = loadstring(serpent.dump(a, {sparse = false, nocode = true}))()
 assert(pcall(function() _a.z() end) == false, "nocode replaces functions with dummy errors: failed")
 assert(#(_a.list) == #(a.list), "size of array part stays the same: failed")
+
+local diffable = {sortkeys = true, comment = false, nocode = true, indent = ' '}
+assert(serpent.block(a, diffable) == serpent.block(_a, diffable),
+ "block(a) == block(copy_of_a): failed")
+
+-- test maxlevel
+_a = loadstring(serpent.dump(a, {sparse = false, nocode = true, maxlevel = 1}))()
+assert(#(_a.list) == 0, "nested table 1 is empty with maxlevel=1: failed")
+assert(#(_a[true]) == 0, "nested table 2 is empty with maxlevel=1: failed")