I would not call that 'syntax tree', but something like 'token tree' or 'nested token lists'. For the Lisp reader the form (+ 1 2) in (first '(+ 1 2)) and

I would not call that 'syntax tree', but something like 'token tree' or 'nested token lists'. For the Lisp reader the form (+ 1 2) in (first '(+ 1 2)) and (* (+ 1 2) 3) looks the same. It has no idea that the first is data or code represented as data. It has also no idea that the second is actual code and which syntax (here some prefix syntax) it uses. All we represent is a bunch of tokens in nested lists.