Thanks for your nice intro in picolisp coding!
I would like to point at the fact, that
(print (1 "abc" (d e (f)) 999 ghi))
does only work without a single quote (') in front of the list because of a picolisp distinctive feature: a list starting with a number will not evaluate as a function call - or exactly spoken: "if the CAR of the list is a number, the whole list is returned as it is"
To print lists not starting with a number, one has to add the single quote in front of the list to escape evaluation of the list as a function call.
(print '("abc" (d e (f)) 999 ghi))
(otherwise picolisp will not print the list and respond: "abc" -- undefined )
In my early days with picolisp I got confused by different code examples with lists being quoted sometimes and being not quoted other times - it took a while until I really checked that simple rule above :-)