The second novel feature is the use of a nonstandard assembly language, BREVITY, for the 6502 microprocessor. BREVITY was specifically designed to permit efficient assembly language programming on 6502 systems containing no more than 12-16K bytes of primary memory and only tape cassette secondary storage. It is a clean, precisely defined, readable, and easily learned language with some unusual features such as two-character (only) labels, one-character (mathematical) opcodes, one-character addressing mode syntax, and locally defined labels. No statement is longer than eight characters and, consequently, a source program is only three to four times as long as the object program it produces. A 300-line BREVITY program can be listed on a single page. For individuals with a 48K-byte primary memory and a floppy disk, the use of BREVITY means there is going to be lots of room left over for the user's systems programs.*

*The BREVITY software development system which is designed in this book has been tailored to run on many of the popular 6502 microcomputer systems and is available for sale. Details on ordering this software are available from T. G. Windeknecht, P.O. Box 232, Rochester, Michigan 48308-0232

Second, it uses BREVITY, a very appropriately named nonstandard assembly language, tor the 6502 microprocessor. BREVITY was specifically designed for maximum efficiency in assembly language programming on 6502 systems containing no more than 12-16K bytes of primary memory and only tape cassette secondary storage. (Of course, with a 48K byte primary memory and a floppy disk, using BREVITY means there's lots of room left over for your systems programs.) It is precise, readable, and extremely easy to learn and has these novel features:
[*] Two-character labels, one-character mathematical opcodes, one-character addressing mode syntax, and locally defined labels.
[*] No statement is longer than eight characters — so a source program is only three or four times as long as the object program it produces.
[*] A 300 line BREVITY program can be listed on a single page

(It was very easy to find a dodgy PDF download for the book, but I think we tend not to share such links here.)

Because of the rather terse, mathematical nature of its statements, we call our 6502 assembly language BREVITY.
Everything about BREVITY is designed to minimize memory requirements and typing.
However, we also feel that terse, mathematical statements promote readability, which is one of
the more difficult things to achieve in assembly language design.
In small installations, no comment statements are permitted in BREVITY programs.
Instead, some standard methods of developing separate documentation are proposed.
In BREVITY, labels denoting hexadecimal addresses consist of precisely two alphanumeric characters.
Since this means there are only 36 x 36 = 1296 possible labels in all, we allow labels to be redefined within a program.
This has a profound effect on (1) assembler design and (2) reducing overall memory requirements.
These topics are taken up in Chapter 6.
No arithmetic expressions involving labels and addresses are used in BREVITY.
We believe that such expressions are not warranted in terms of their utility in systems programming.
Since there are no comment statements nor arithmetic expressions, the longest statement in BREVITY is only eight characters.
Specific operations and conditions are generally represented by a one-character symbol, as in mathematics.
In almost every case, the one-character symbol chosen is already semantically related to the operation or
condition denoted in applied mathematics, at least in the author's experience.
Hopefully, these associations will be palatable to the reader as well.

That's awfully cryptic. It's apparently actual source code. Is there an interpretation?

I think this would be a very nice resurrection project.