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diff --git a/libbuild2/config/module.hxx b/libbuild2/config/module.hxx
new file mode 100644
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+++ b/libbuild2/config/module.hxx
@@ -0,0 +1,93 @@
+// file      : libbuild2/config/module.hxx -*- C++ -*-
+// copyright : Copyright (c) 2014-2019 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#ifndef LIBBUILD2_CONFIG_MODULE_HXX
+#define LIBBUILD2_CONFIG_MODULE_HXX
+
+#include <map>
+
+#include <libbutl/prefix-map.mxx>
+
+#include <libbuild2/types.hxx>
+#include <libbuild2/utility.hxx>
+
+#include <libbuild2/module.hxx>
+#include <libbuild2/variable.hxx>
+
+namespace build2
+{
+  namespace config
+  {
+    // An ordered list of modules each with an ordered list of list of
+    // config.* variables and their "save flags" (see save_variable()) that
+    // are used (as opposed to just being specified) in this configuration.
+    // Populated by the config utility functions (required(), optional())
+    // and saved in the order populated.
+    //
+    struct saved_variable
+    {
+      reference_wrapper<const variable> var;
+      uint64_t flags;
+    };
+
+    struct saved_variables: vector<saved_variable>
+    {
+      // Normally each module only have a handful of config variables and we
+      // only do this during configuration so for now we do linear search
+      // instead of adding a map.
+      //
+      const_iterator
+      find (const variable& var) const
+      {
+        return std::find_if (
+          begin (),
+          end (),
+          [&var] (const saved_variable& v) {return var == v.var;});
+      }
+    };
+
+    struct saved_modules: butl::prefix_map<string, saved_variables, '.'>
+    {
+      // Priority order with INT32_MIN being the highest. Modules with the
+      // same priority are saved in the order inserted.
+      //
+      // Generally, the idea is that we want higher-level modules at the top
+      // of the file since that's the configuration that we usualy want to
+      // change. So we have the following priority bands/defaults:
+      //
+      // 101-200/150 - code generators (e.g., yacc, bison)
+      // 201-300/250 - compilers (e.g., C, C++),
+      // 301-400/350 - binutils (ar, ld)
+      //
+      std::multimap<std::int32_t, const_iterator> order;
+
+      iterator
+      insert (string name, int prio = 0)
+      {
+        auto p (emplace (move (name), saved_variables ()));
+
+        if (p.second)
+          order.emplace (prio, p.first);
+
+        return p.first;
+      }
+    };
+
+    struct module: module_base
+    {
+      config::saved_modules saved_modules;
+
+      void
+      save_variable (const variable&, uint64_t flags = 0);
+
+      void
+      save_module (const char* name, int prio = 0);
+
+      static const string name;
+      static const uint64_t version;
+    };
+  }
+}
+
+#endif // LIBBUILD2_CONFIG_MODULE_HXX