Add a value at the start of a list.

[StephenG1CMZ]

CONCAT({1,2},{3,4}) -> {1,2,3,4} seems ok for lists, which in PPL have braces.
The [] are matrices or vectors, though in cas I believe they can be mixed.
Some XCAS commands are implemented, but not all.
I have a collection of useful list functions here:
https://www.hpmuseum.org/forum/thread-94...t=list+api

(If you need to use [], there may be be a way, or you can convert to a list and then back again).