(reverse Polish) notation.

Historical note: AOS in sense 1 was the name of a {PDP-10}
instruction that took any memory location in the computer and added
1 to it; AOS meant `Add One and do not Skip'. Why, you may ask,
does the `S' stand for `do not Skip' rather than for `Skip'? Ah,
here was a beloved piece of PDP-10 folklore. There were eight such
instructions: AOSE added 1 and then skipped the next instruction
if the result was Equal to zero; AOSG added 1 and then skipped if
the result was Greater than 0; AOSN added 1 and then skipped
if the result was Not 0; AOSA added 1 and then skipped Always;
and so on. Just plain AOS didn't say when to skip, so it never
skipped.

For similar reasons, AOJ meant `Add One and do not Jump'. Even
more bizarre, SKIP meant `do not SKIP'! If you wanted to skip the
next instruction, you had to say `SKIPA'. Likewise, JUMP meant
`do not JUMP'; the unconditional form was JUMPA. However, hackers
never did this. By some quirk of the 10's design, the {JRST}
(Jump and ReSTore flag with no flag specified) was actually faster
and so was invariably used. Such were the perverse mysteries of
assembler programming.

:app: /ap/ n. Short for `application program', as opposed to a
systems program. What systems vendors are forever chasing
developers to create for their environments so they can sell more
boxes. Hackers tend not to think of the things they themselves run
as apps; thus, in hacker parlance the term excludes compilers,
program editors, games, and messaging systems, though a user would
consider all those to be apps. Oppose {tool}, {operating