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diff --git a/nselib/strict.lua b/nselib/strict.lua
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+---
+-- Strict declared global library. Checks for undeclared global variables
+-- during runtime execution.
+--
+-- This module places the <code>strict</code> function in the global
+-- environment. The strict function allows a script to add runtime checking so
+-- that undeclared globals cause an error to be raised. This is useful for
+-- finding accidental use of globals when local was intended.
+--
+-- A global variable is considered "declared" if the script makes an assignment
+-- to the global name (even <code>nil</code>) in the file scope.
+--
+-- @class module
+-- @name strict
+-- @copyright Same as Nmap--See https://nmap.org/book/man-legal.html
+
+local debug = require "debug"
+
+local error = error;
+local getmetatable = getmetatable;
+local rawset = rawset;
+local rawget = rawget;
+local setmetatable = setmetatable;
+local type = type;
+
+local getinfo = debug.getinfo;
+
+_ENV = {};
+
+local function what ()
+  local d = getinfo(3, "S");
+  return d and d.what or "C";
+end
+
+--- The strict function.
+--
+-- This function adds runtime checking to the global environment for use of
+-- undeclared globals. A global is 'undeclared' if not assigned in the file
+-- (script) scope previously. An error will be raised on use of an undeclared
+-- global.
+--
+-- This function should be passed last to stdnse.module in order to allow
+-- other environment option functions (e.g. stdnse.seeall) to change the
+-- environment first. This is important for allowing globals outside the
+-- library (in _G) to be indexed.
+--
+-- @see stdnse.module
+-- @usage
+--  _ENV = stdnse.module(name, require "strict");
+-- @param env The environment to modify.
+local function strict (env)
+  local mt = getmetatable(env) or setmetatable(env, {}) and getmetatable(env);
+  local _newindex, _index = mt.__newindex, mt.__index;
+
+  mt.__declared = {};
+
+  function mt.__newindex (t, n, v)
+    if type(_newindex) == "function" then
+      _newindex(t, n, v); -- hook it
+    end
+    if not mt.__declared[n] then
+      local w = what();
+      if w ~= "main" and w ~= "C" then
+        error("assign to undeclared variable '"..n.."'", 2);
+      end
+      mt.__declared[n] = true;
+    end
+    rawset(t, n, v);
+  end
+
+  function mt.__index (t, n)
+    if type(_index) == "function" then
+      local v = _index(t, n); -- hook it
+      if v ~= nil then return v end
+    elseif _index ~= nil then
+      local v = _index[n];
+      if v ~= nil then return v end
+    end
+    if not mt.__declared[n] and what() ~= "C" then
+      error("variable '"..n.."' is not declared", 2);
+    end
+    return rawget(t, n);
+  end
+
+  return env;
+end
+
+return strict;