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LISP

lisp is confusing because the AST and the values that result
from evaluating expressions are often considered to be the same
data structure.

in this implementation expressions (and values) consist of

456 -- numbers
abc -- symbols
(a b c) -- s-expressions

except for special forms, evaluating an s-expression will be
interpreted as function application. when the first element of
the list is not a lambda, an error occurs.

using the repl provided, try the following examples

8
(+ 2 7)
(< 2 7)
(= abc abc)
(list 1 2 3 4)
(let (f 3) (f (lambda (x) (+ x 1))))
(cond ((< 4 3) barf) ((> 3 4) puke))
(car (list 1 2 3))
(cdr (list 1 2 3))
(cons a (list b c))
(syscall 13 foo)
(car ())