Lisp variables can have, in addition to a value, a list of associated pairs and flags called a property list (often abbreviated to plist). Most of the information kept both by REDUCE and REDTEN is in the form of property lists attached to various lisp identifiers. While the REDTEN user generally does not need to access a property list, there may be times when it is useful to make corrections or alterations directly. It is also useful to know how to examine the property list either for the sake of curiosity, to fix a problem, or to ``cheat''.
Although referred to as a list, the property list is more conveniently thought of as a series of keyword-value pairs, which can be accessed with the lisp functions put() and get(), and a set of flags (which are either present or not) that are accessed with the functions flag() and flagp(). Properties and flags can be deleted with the functions remprop() and remflag(). The entire property list can be accessed or replaced in REDTEN with the functions prop() and setprop(), these are defined in terms of standard lisp functions that vary from system to system (see appendix F). The formats of the calls to these functions is shown in C.1.