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 user
Note 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.