+ /** Save (serialize) the object into archive node.
+ *
+ * Losely speaking, this method turns an expression into a byte
+ * stream (which can be saved and restored later on, or sent via
+ * network, etc.)
+ */
+ virtual void archive(archive_node& n) const;
+ /** Load (deserialize) the object from an archive node.
+ *
+ * @note This method is essentially a constructor. However,
+ * constructors can't be virtual. So, if unarchiving routines
+ * are implemented as constructors one would need to define such
+ * a constructor in every class, even if all it does is simply
+ * calling constructor of a superclass.
+ */
+ virtual void read_archive(const archive_node& n, lst& syms); // no const
+