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diff --git a/libbuild2/build/script/script.hxx b/libbuild2/build/script/script.hxx
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+++ b/libbuild2/build/script/script.hxx
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+// file      : libbuild2/build/script/script.hxx -*- C++ -*-
+// license   : MIT; see accompanying LICENSE file
+
+#ifndef LIBBUILD2_BUILD_SCRIPT_SCRIPT_HXX
+#define LIBBUILD2_BUILD_SCRIPT_SCRIPT_HXX
+
+#include <libbuild2/types.hxx>
+#include <libbuild2/forward.hxx>
+#include <libbuild2/utility.hxx>
+
+#include <libbuild2/variable.hxx>
+
+#include <libbuild2/script/script.hxx>
+
+namespace build2
+{
+  namespace build
+  {
+    namespace script
+    {
+      using build2::script::line;
+      using build2::script::line_type;
+      using build2::script::command_expr;
+
+      // Once parsed, the script can be executed in multiple threads with the
+      // state (variable values, etc) maintained by the environment object.
+      //
+      class script
+      {
+      public:
+        build2::script::lines lines;
+
+        variable_pool var_pool;
+        mutable shared_mutex var_pool_mutex;
+
+        const variable& primary_target_var; // $>
+
+        location start_loc;
+        location end_loc;
+
+        script ();
+
+        script (script&&) = delete;
+        script (const script&) = delete;
+        script& operator= (script&&) = delete;
+        script& operator= (const script&) = delete;
+      };
+
+      class environment: public build2::script::environment
+      {
+      public:
+        environment (const script&, const target& primary_target);
+
+        environment (environment&&) = delete;
+        environment (const environment&) = delete;
+        environment& operator= (environment&&) = delete;
+        environment& operator= (const environment&) = delete;
+
+      public:
+        const build::script::script& script;
+
+        // Note that if we pass the variable name as a string, then it will
+        // be looked up in the wrong pool.
+        //
+        variable_map vars;
+
+        const target& primary_target;
+
+        virtual void
+        set_variable (string&& name, names&&, const string& attrs) override;
+
+        // Variables.
+        //
+      public:
+        // Lookup the variable starting from this environment, then the
+        // primary target, and then outer buildfile scopes.
+        //
+        using lookup_type = build2::lookup;
+
+        lookup_type
+        lookup (const variable&) const;
+
+        lookup_type
+        lookup (const string&) const;
+
+        // As above but only look for buildfile variables.
+        //
+        lookup_type
+        lookup_in_buildfile (const string&) const;
+
+        // Return a value suitable for assignment. If the variable does not
+        // exist in this environment's map, then a new one with the NULL value
+        // is added and returned. Otherwise the existing value is returned.
+        //
+        value&
+        assign (const variable& var) {return vars.assign (var);}
+
+        // Return a value suitable for append/prepend. If the variable does
+        // not exist in this environment's map, then outer scopes are searched
+        // for the same variable. If found then a new variable with the found
+        // value is added to the environment and returned. Otherwise this
+        // function proceeds as assign() above.
+        //
+        value&
+        append (const variable&);
+      };
+    }
+  }
+}
+
+#endif // LIBBUILD2_BUILD_SCRIPT_SCRIPT_HXX