This appendix describes the design of the program xtract and the macro language used to extract visibility and auxiliary data from GMRT LTA database. Section A.1 is intended for "plain" users of the program. The xtract macro language parser is also available as a stand-alone library, which can be used in other applications. The internal design of the library is described in section B.3 and is targeted for more enterprising users, who will find if useful to extend this program. Section B.4 describes the application programmers interface (API) of the library. Section B.5 describes the mechanism to extend the list of data/parameters which can be extracted.