diff --git a/src/Input/input.hxx b/src/Input/input.hxx
index bc7bb55c4..c046ba69d 100644
--- a/src/Input/input.hxx
+++ b/src/Input/input.hxx
@@ -61,6 +61,12 @@ public:
   FGBinding ();
 
 
+  /**
+   * Copy constructor.
+   */
+  FGBinding (const FGBinding &binding);
+
+
   /**
    * Convenience constructor.
    *
@@ -126,10 +132,11 @@ public:
 
 
 private:
-  void _fire (const SGPropertyNode *arg) const;
   string _command_name;
   SGCommandMgr::command_t _command;
-  const SGPropertyNode * _arg;
+  mutable SGPropertyNode * _arg;
+  mutable SGPropertyNode * _setting;
+  mutable SGCommandState * _command_state;
 };