This is a Common Lisp library for creating trees. This undoubtedly sounds vague so at this point you should probably visit the manual and have a look at a few examples. Here I will give the general motivation. When writing code for macros , the macro code generally constructs a list which is the macro expansion. Sometimes you can do this using MAPCAR and similar. But often I found myself in a situation where I constructed a partial list and then , as I was parsing the code given in an invocation of the macro , I gradually wanted to add more elements to the end of the list. Using the standard APPEND for this gives quadratic performance. So what I was doing was to PUSH elements to the front of the list and , when I had created the whole expansion , I would do (reverse list) .With this approach the code I was writing often looked unnatural to me. In situations where one can construct the expansion list in one loop and it's not convenient to use MAPCAR {or similar} , many people would use LOOP .I don't like LOOP either so I needed a different solution hence I wrote this library. The main idea is that you can append to the list you are constructing in constant time rather than having to traverse the whole list from the start. It's not just for macro expansions obviously but I almost exlusively use it for that because , in every other situation where I need a sequence , I'm much more likely to use a vector than a list. In order to use the library you unpack the tar file {everything will expand under the same directory as the tar file itself} , start your favorite Common Lisp implementation {I have tested this on SBCL , clisp and ECL .I do not believe I have introduced any implementation specific code} , do (load "tests") and , if you don't get any errors , you're good to go. The manual is detailed and should tell you everything else you need to know. You may also find it informative to read the following thread which discusses some other approaches to the issue : Newsgroups: comp.lang.lisp Subject: Looking for a specific piece of code Message-ID: <6ASGDShANq9NtnAYObpTxXm0AAqrK@bongo-ra.co> Date: Wed, 19 Dec 2018 13:09:20 GMT or http://al.howardknight.net/?ID=165980231900 . The other message IDs of the thread are <87h8f94ozu.fsf@hubble.informatimago.com> <6ASGDShANq9NtnAYObpTxXm0AAqr7@bongo-ra.co> LICENSE Public domain i.e. you can do anything you want with the code and the documentation. Spiros Bousbouras August 2022