All list, and related, structure definitions use integer type fields to identify referenced data structures. A current list of defined data types and their associated meanings is given in the Appendix. They fall into the following categories which are grouped in terms of their numeric values
1......99 used to define system data structures 100...199 used to define math types 200...299 used to define Tv types 300...999 used to define Tina data structures 1000..... available to the userNote that the NULL type 0 is not used explicitly. This allows the type NULL to be used both as a catch all and in situations where it is thought unnecessary to specify the type of the elements.