Implementation of new closed orbit correction procedures in the MAD-X program

The possibility to correct a closed orbit is a basic functionality in the MAD program. During the rewrite leading to the MAD-X version, the closed orbit correction procedures have been completely re-written and substantially extended. Additional algorithms have been added and problems with existing algorithms have been solved. Furthermore, new options allow more sophisticated studies and better control of the correction procedures. In this note I shall describe these new features and their raison d'etre. Most new features are the result of the immediate needs emerging from the extensive use of the program in realistic simulations. Some features which were practically never used have been dropped.