key of referents ([ numeric-expression] | this referent)
Obtains the key of a particular referent. The numeric-expression represents the position of the to-be-accessed referent on the referents shelf.
In a simple program, the "n"th value on the referents shelf is the "n"th mentioned in the program. However, if a stream bound to a referent is discarded while open, that referent is removed from the shelf, and the positional values of some of the referents may change.