Common Lisp Series
cs.cmu.edu
cs.cmu.edu
The section in the preface that GLS' note refers to reads:
"X3J13 expressed interest in the other two approaches (series and generators), but the consensus as of January 1989 was that these other approaches were not yet sufficiently mature or in sufficiently widespread use to warrant inclusion in the draft Common Lisp standard at that time. However, the subcommittee was directed to continue work on these approaches and X3J13 is open to the possibility of standardizing them at a later date. Please note that I do not wish the prejudge the question of whether X3J13 will ever choose to make the other two proposals the subject of standardization. Nevertheless, I have chosen to include them in the second edition, in cooperation with Dr. Richard C. Waters, as appendices A and B, in order to make these ideas available to the Lisp community. In my judgement these proposals address an area of language design not otherwise covered by Common Lisp and are likely to have practical value even if they are never adopted as part of a formal standard."
They didn't end up making it into the Common Lisp standard, but they're on Quicklisp, so getting (both of) them is as simple as (ql:quickload :series), and then you can load them with (series::install).