On 64-bit systems, try the following in REPL.
(+ -9223372036854775808 -1)
(+ 9223372036854775807 1)
Luckly, this is caught with UBSan.
You can see the details of this bug in the README of https://codeberg.org/iacore/newerlisp
P.S. does anyone know an existing fork of newLISP?
This seems to be how newLISP works