diff --git a/containers.lua b/containers.lua
index e706831..cd3e7c0 100644
--- a/containers.lua
+++ b/containers.lua
@@ -1,4 +1,4 @@
-local List, Dict
+local List, Dict, Undict, _undict_mt, _dict_mt
 local insert, remove, concat
 do
   local _obj_0 = table
@@ -226,6 +226,23 @@ local _list_mt = {
         return _accum_0
       end)())
     end,
+    copy = function(self)
+      return List((function()
+        local _accum_0 = { }
+        local _len_0 = 1
+        for i = 1, #self do
+          _accum_0[_len_0] = self[i]
+          _len_0 = _len_0 + 1
+        end
+        return _accum_0
+      end)())
+    end,
+    reverse = function(self)
+      local n = #self
+      for i = 1, math.floor(n / 2) do
+        self[i], self[n - i + 1] = self[n - i + 1], self[i]
+      end
+    end,
     reversed = function(self)
       return List((function()
         local _accum_0 = { }
@@ -236,6 +253,49 @@ local _list_mt = {
         end
         return _accum_0
       end)())
+    end,
+    sort = function(self)
+      return table.sort(self)
+    end,
+    sort_by = function(self, fn)
+      local keys = setmetatable({ }, {
+        __index = function(self, k)
+          local key = fn(k)
+          self[k] = key
+          return key
+        end
+      })
+      return table.sort(self, function(a, b)
+        return keys[a] <= keys[b]
+      end)
+    end,
+    sorted = function(self)
+      local c = self:copy()
+      c:sort()
+      return c
+    end,
+    sorted_by = function(self, fn)
+      local c = self:copy()
+      c:sort_by(fn)
+      return c
+    end,
+    filter_by = function(self, keep)
+      local deleted = 0
+      for i = 1, #self do
+        if not (keep(self[i])) then
+          deleted = deleted + 1
+        elseif deleted > 0 then
+          self[i - deleted] = self[i]
+        end
+      end
+      for i = #self - deleted + 1, #self do
+        self[i] = nil
+      end
+    end,
+    filtered_by = function(self, keep)
+      local c = self:copy()
+      c:filter_by(keep)
+      return c
     end
   },
   __newindex = function(self, k, v)
@@ -262,7 +322,91 @@ List = function(t)
     return error("Unsupported List type: " .. type(t))
   end
 end
-local _dict_mt = {
+local compliments = setmetatable({ }, {
+  __mode = 'k'
+})
+_undict_mt = {
+  __index = function(self, k)
+    return not compliments[self][k] and true or nil
+  end,
+  __newindex = function(self, k, v)
+    if k then
+      compliments[self][k] = nil
+    else
+      compliments[self][k] = true
+    end
+  end,
+  __eq = function(self, other)
+    if not (type(other) == 'table' and getmetatable(other) == getmetatable(self)) then
+      return false
+    end
+    return compliments[self] == compliments[other]
+  end,
+  __len = function(self)
+    return math.huge
+  end,
+  __tostring = function(self)
+    return "~" .. _dict_mt.__tostring(compliments[self])
+  end,
+  as_nomsu = function(self)
+    return "~" .. _dict_mt.as_nomsu(compliments[self])
+  end,
+  as_lua = function(self)
+    return "~" .. __dict_mt.as_lua(compliments[self])
+  end,
+  __band = function(self, other)
+    if getmetatable(other) == _undict_mt then
+      return Undict(_dict_mt.__bor(compliments[self], compliments[other]))
+    else
+      return _dict_mt.__band(other, self)
+    end
+  end,
+  __bor = function(self, other)
+    if getmetatable(other) == _undict_mt then
+      return Undict(_dict_mt.__band(compliments[self], compliments[other]))
+    else
+      return Undict((function()
+        local _tbl_0 = { }
+        for k, v in pairs(compliments[self]) do
+          if not other[k] then
+            _tbl_0[k] = v
+          end
+        end
+        return _tbl_0
+      end)())
+    end
+  end,
+  __bxor = function(self, other)
+    if getmetatable(other) == _undict_mt then
+      return _dict_mt.__bxor(compliments[self], compliments[other])
+    else
+      return Undict(_dict_mt.__band(other, self))
+    end
+  end,
+  __bnot = function(self)
+    return Dict((function()
+      local _tbl_0 = { }
+      for k, v in pairs(compliments[self]) do
+        _tbl_0[k] = v
+      end
+      return _tbl_0
+    end)())
+  end
+}
+Undict = function(d)
+  local u = setmetatable({ }, _undict_mt)
+  compliments[u] = Dict((function()
+    local _tbl_0 = { }
+    for k, v in pairs(d) do
+      if v then
+        _tbl_0[k] = true
+      end
+    end
+    return _tbl_0
+  end)())
+  return u
+end
+_dict_mt = {
   __type = "a Dict",
   __eq = function(self, other)
     if not (type(other) == 'table' and getmetatable(other) == getmetatable(self)) then
@@ -292,7 +436,7 @@ local _dict_mt = {
       local _accum_0 = { }
       local _len_0 = 1
       for k, v in pairs(self) do
-        _accum_0[_len_0] = "." .. tostring(k) .. " = " .. tostring(v)
+        _accum_0[_len_0] = v == true and "." .. as_nomsu(k) or "." .. tostring(k) .. " = " .. tostring(v)
         _len_0 = _len_0 + 1
       end
       return _accum_0
@@ -303,7 +447,7 @@ local _dict_mt = {
       local _accum_0 = { }
       local _len_0 = 1
       for k, v in pairs(self) do
-        _accum_0[_len_0] = "." .. tostring(as_nomsu(k)) .. " = " .. tostring(as_nomsu(v))
+        _accum_0[_len_0] = v == true and "." .. as_nomsu(k) or "." .. tostring(as_nomsu(k)) .. " = " .. tostring(as_nomsu(v))
         _len_0 = _len_0 + 1
       end
       return _accum_0
@@ -320,11 +464,22 @@ local _dict_mt = {
       return _accum_0
     end)(), ", ") .. "}"
   end,
+  as_list = function(self)
+    return List((function()
+      local _accum_0 = { }
+      local _len_0 = 1
+      for k, v in pairs(self) do
+        _accum_0[_len_0] = k
+        _len_0 = _len_0 + 1
+      end
+      return _accum_0
+    end)())
+  end,
   __band = function(self, other)
     return Dict((function()
       local _tbl_0 = { }
       for k, v in pairs(self) do
-        if other[k] ~= nil then
+        if other[k] then
           _tbl_0[k] = v
         end
       end
@@ -332,6 +487,9 @@ local _dict_mt = {
     end)())
   end,
   __bor = function(self, other)
+    if getmetatable(other) == _undict_mt then
+      return _undict_mt.__bor(other, self)
+    end
     local ret = Dict((function()
       local _tbl_0 = { }
       for k, v in pairs(self) do
@@ -347,6 +505,9 @@ local _dict_mt = {
     return ret
   end,
   __bxor = function(self, other)
+    if getmetatable(other) == _undict_mt then
+      return _undict_mt.__bxor(other, self)
+    end
     local ret = Dict((function()
       local _tbl_0 = { }
       for k, v in pairs(self) do
@@ -363,6 +524,7 @@ local _dict_mt = {
     end
     return ret
   end,
+  __bnot = Undict,
   __add = function(self, other)
     local ret = Dict((function()
       local _tbl_0 = { }
diff --git a/containers.moon b/containers.moon
index 6fa9ecb..06fbccc 100644
--- a/containers.moon
+++ b/containers.moon
@@ -1,5 +1,5 @@
 -- This file contains container classes, i.e. Lists and Dicts, plus some extended string functionality
-local List, Dict
+local List, Dict, Undict, _undict_mt, _dict_mt
 {:insert,:remove,:concat} = table
 
 as_nomsu = =>
@@ -88,8 +88,42 @@ _list_mt =
             start = (n+1-start) if start < 0
             stop = (n+1-stop) if stop < 0
             return List[@[i] for i=start,stop]
-
+        copy: => List[@[i] for i=1,#@]
+        reverse: =>
+            n = #@
+            for i=1,math.floor(n/2)
+                @[i], @[n-i+1] = @[n-i+1], @[i]
         reversed: => List[@[i] for i=#@,1,-1]
+        sort: => table.sort(@)
+        sort_by: (fn)=>
+            keys = setmetatable {},
+                __index: (k)=>
+                    key = fn(k)
+                    @[k] = key
+                    return key
+            table.sort(@, (a,b)->keys[a] <= keys[b])
+        sorted: =>
+            c = @copy!
+            c\sort!
+            return c
+        sorted_by: (fn)=>
+            c = @copy!
+            c\sort_by(fn)
+            return c
+        filter_by: (keep)=>
+            deleted = 0
+            for i=1,#@
+                unless keep(@[i])
+                    deleted += 1
+                elseif deleted > 0
+                    @[i-deleted] = @[i]
+            for i=#@-deleted+1,#@
+                @[i] = nil
+
+        filtered_by: (keep)=>
+            c = @copy!
+            c\filter_by(keep)
+            return c
 
     -- TODO: remove this safety check to get better performance?
     __newindex: (k,v)=>
@@ -109,6 +143,52 @@ List = (t)->
         return l
     else error("Unsupported List type: "..type(t))
 
+
+compliments = setmetatable({}, {__mode:'k'})
+_undict_mt =
+    __type: "an Inverse Dict"
+    __index: (k)=> not compliments[@][k] and true or nil
+    __newindex: (k,v)=>
+        if k
+            compliments[@][k] = nil
+        else
+            compliments[@][k] = true
+    __eq: (other)=>
+        unless type(other) == 'table' and getmetatable(other) == getmetatable(@)
+            return false
+        return compliments[@] == compliments[other]
+    __len: => math.huge
+    __tostring: => "~".._dict_mt.__tostring(compliments[@])
+    as_nomsu: => "~".._dict_mt.as_nomsu(compliments[@])
+    as_lua: => "~"..__dict_mt.as_lua(compliments[@])
+    __band: (other)=>
+        if getmetatable(other) == _undict_mt
+            -- ~{x,y} & ~{y,z} == ~{x,y,z} == ~({x,y} | {y,z})
+            Undict(_dict_mt.__bor(compliments[@], compliments[other]))
+        else
+            -- ~{x,y} & {y,z} == {z} == {y,z} & ~{x,y}
+            _dict_mt.__band(other, @)
+    __bor: (other)=>
+        if getmetatable(other) == _undict_mt
+            -- ~{x,y} | ~{y,z} == ~{y} = ~({x,y} & {y,z})
+            Undict(_dict_mt.__band(compliments[@], compliments[other]))
+        else
+            -- ~{x,y} | {y,z} == ~{z} = ~({y,z} & ~{x,y})
+            Undict{k,v for k,v in pairs(compliments[@]) when not other[k]}
+    __bxor: (other)=>
+        if getmetatable(other) == _undict_mt
+            -- ~{x,y} ^ ~{y,z} == {x,z} = {x,y} ^ {y,z}
+            _dict_mt.__bxor(compliments[@], compliments[other])
+        else
+            -- ~{x,y} ^ {y,z} == ~{x} = ~({x,y} & ~{y,z})
+            Undict(_dict_mt.__band(other, @))
+    __bnot: => Dict{k,v for k,v in pairs(compliments[@])}
+
+Undict = (d)->
+    u = setmetatable({}, _undict_mt)
+    compliments[u] = Dict{k,true for k,v in pairs(d) when v}
+    return u
+
 _dict_mt =
     __type: "a Dict"
     __eq: (other)=>
@@ -124,24 +204,31 @@ _dict_mt =
         for _ in pairs(@) do n += 1
         return n
     __tostring: =>
-        "{"..concat([".#{k} = #{v}" for k,v in pairs @], ", ").."}"
+        "{"..concat([v == true and "."..as_nomsu(k) or ".#{k} = #{v}" for k,v in pairs @], ", ").."}"
     as_nomsu: =>
-        "{"..concat([".#{as_nomsu(k)} = #{as_nomsu(v)}" for k,v in pairs @], ", ").."}"
+        "{"..concat([v == true and "."..as_nomsu(k) or ".#{as_nomsu(k)} = #{as_nomsu(v)}" for k,v in pairs @], ", ").."}"
     as_lua: =>
         "a_Dict{"..concat(["[ #{as_lua(k)}]= #{as_lua(v)}" for k,v in pairs @], ", ").."}"
+    as_list: => List[k for k,v in pairs(@)]
     __band: (other)=>
-        Dict{k,v for k,v in pairs(@) when other[k] != nil}
+        Dict{k,v for k,v in pairs(@) when other[k]}
     __bor: (other)=>
+        if getmetatable(other) == _undict_mt
+            return _undict_mt.__bor(other, @)
         ret = Dict{k,v for k,v in pairs(@)}
         for k,v in pairs(other)
             if ret[k] == nil then ret[k] = v
         return ret
     __bxor: (other)=>
+        if getmetatable(other) == _undict_mt
+            return _undict_mt.__bxor(other, @)
         ret = Dict{k,v for k,v in pairs(@)}
         for k,v in pairs(other)
             if ret[k] == nil then ret[k] = v
             else ret[k] = nil
         return ret
+    __bnot: Undict
+
     __add: (other)=>
         ret = Dict{k,v for k,v in pairs(@)}
         for k,v in pairs(other)
@@ -154,6 +241,7 @@ _dict_mt =
             if ret[k] == nil then ret[k] = -v
             else ret[k] -= v
         return ret
+
 Dict = (t)->
     if type(t) == 'table'
         return setmetatable(t, _dict_mt)