diff --git a/Lua/pt.lua b/Lua/pt.lua
new file mode 100644
index 0000000..5f16f1b
--- /dev/null
+++ b/Lua/pt.lua
@@ -0,0 +1,143 @@
+require 'extra'
+
+local pt = {}
+pt.__index = pt
+setmetatable(pt, pt)
+
+function rawstr(v)
+    if v == nil then return 'nil' end
+    local mt = getmetatable(v)
+    local ts = mt and rawget(mt, '__tostring')
+    if not ts then return tostring(v) end
+    mt.__tostring = nil
+    local s = tostring(v)
+    mt.__tostring = ts
+    return s
+end
+
+function getaddr(t)
+    return rawstr(t):sub(#type(t) + 3)
+end
+
+function copy(t)
+    -- shallow copy
+    if type(t) ~= 'table' then return end
+    local new = {}
+    for key, value in pairs(t) do
+        new[key] = value
+    end
+    return new
+end
+
+function pt.__call(pt, args)
+    -- print a table as semi-valid YAML
+    -- with references to prevent recursion/duplication
+    local t = args.table or args[1]
+    local self = {}
+    setmetatable(self, pt)
+    self.seen = copy(args.seen) or {}
+    self.skipped = copy(args.skipped) or {}
+    self.seen_elsewhere = args.seen or {}
+    self.depth = args.depth or 16
+    self.writer = args.writer or io.write
+    self.skeleton = args.skeleton or false
+    self.queued = {}
+    self:inner('__root__', t, '')
+    return self.seen
+end
+
+function pt:write(...)
+    self.writer(...)
+end
+
+function pt.safekey(k)
+    if type(k) == 'table' then
+        return 't'..getaddr(k)
+    end
+    local s = tostring(k)
+    s = s:gsub('[\r\n]', '')
+    return s:find('[^%w_]') and ('%q'):format(s) or s
+end
+
+function pt.safeval(v, indent)
+    if type(v) == 'function' then
+        return 'f'..getaddr(v)
+    end
+    local s = tostring(v)
+    if type(v) == 'number' then
+        return s
+    end
+    s = s:find('[\r\n]') and ('\n'..s):gsub('[\r\n]', '\n'..indent..' ') or s
+    --local safe = ('%q'):format(s)
+    --return s == safe:sub(2, -2) and s or safe
+    -- TODO: finish matching valid characters
+    return s:find('[^%w_()[]{}.]') and ('%q'):format(s) or s
+end
+
+function pt:inner(k, v, indent)
+    if type(v) ~= 'table' then
+        if self.skeleton then return end
+        self:write(indent, pt.safekey(k), ': ')
+        self:write(pt.safeval(v, indent), '\n')
+        return
+    end
+
+    local addr = getaddr(v)
+    self:write(indent, pt.safekey(k))
+
+    if #indent > self.depth or self.skipped[addr] then
+        --self.skipped[addr] = true -- TODO: extra logics
+        self:write(': #t', addr, '\n')
+        return
+    end
+
+    if self.seen[addr] or self.queued[addr] then
+        self:write(': *t', addr, self.seen_elsewhere[addr] and ' #\n' or '\n')
+        return
+    end
+
+    self.seen[addr] = true
+
+    self:write(': &t', addr, '\n')
+    self:outer(v, indent..' ')
+end
+
+function pt:outer(t, indent)
+    if type(t) ~= "table" then
+        self:write(indent, pt.safeval(t, indent), '\n')
+        return
+    end
+
+    local ours = {}
+    local not_ours = {}
+
+    for k,v in opairs(t) do
+        if type(v) == 'table' then
+            local addr = getaddr(v)
+            if not self.queued[addr] and not self.seen[addr] and not self.skipped[addr] then
+                self.queued[addr] = true
+                ours[k] = v
+            else
+                not_ours[k] = v
+            end
+        else
+            self:inner(k, v, indent)
+        end
+    end
+
+    for k,v in opairs(not_ours) do
+        self:inner(k, v, indent)
+    end
+
+    for k,v in opairs(ours) do
+        self.queued[getaddr(v)] = nil
+        self:inner(k, v, indent)
+    end
+
+    local mt = getmetatable(t)
+    if mt ~= nil then
+        self:inner('__metatable', mt, indent)
+    end
+end
+
+return pt