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diff --git a/libbuild2/in/rule.hxx b/libbuild2/in/rule.hxx
new file mode 100644
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+++ b/libbuild2/in/rule.hxx
@@ -0,0 +1,90 @@
+// file      : libbuild2/in/rule.hxx -*- C++ -*-
+// copyright : Copyright (c) 2014-2019 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#ifndef LIBBUILD2_IN_RULE_HXX
+#define LIBBUILD2_IN_RULE_HXX
+
+#include <libbuild2/types.hxx>
+#include <libbuild2/utility.hxx>
+
+#include <libbuild2/rule.hxx>
+
+#include <libbuild2/in/export.hxx>
+
+namespace build2
+{
+  namespace in
+  {
+    // Preprocess an .in file.
+    //
+    // Note that a derived rule can use the target data pad to cache data
+    // (e.g., in match()) to be used in substitute/lookup() calls.
+    //
+    // Note also that currently this rule ignores the dry-run mode (see
+    // perform_update() for the rationale).
+    //
+    class LIBBUILD2_IN_SYMEXPORT rule: public build2::rule
+    {
+    public:
+      // The rule id is used to form the rule name/version entry in depdb. The
+      // program argument is the pseudo-program name to use in the command
+      // line diagnostics.
+      //
+      rule (string rule_id,
+            string program,
+            char symbol = '$',
+            bool strict = true)
+          : rule_id_ (move (rule_id)),
+            program_ (move (program)),
+            symbol_ (symbol),
+            strict_ (strict) {}
+
+      virtual bool
+      match (action, target&, const string&) const override;
+
+      virtual recipe
+      apply (action, target&) const override;
+
+      virtual target_state
+      perform_update (action, const target&) const;
+
+      // Customization hooks.
+      //
+
+      // Perform prerequisite search.
+      //
+      virtual prerequisite_target
+      search (action,
+              const target&,
+              const prerequisite_member&,
+              include_type) const;
+
+      // Perform variable lookup.
+      //
+      virtual string
+      lookup (const location&,
+              action,
+              const target&,
+              const string& name) const;
+
+      // Perform variable substitution. Return nullopt if it should be
+      // ignored.
+      //
+      virtual optional<string>
+      substitute (const location&,
+                  action,
+                  const target&,
+                  const string& name,
+                  bool strict) const;
+
+    protected:
+      const string rule_id_;
+      const string program_;
+      char symbol_;
+      bool strict_;
+    };
+  }
+}
+
+#endif // LIBBUILD2_IN_RULE_HXX