code:  (define x 10) (- (* x 10) 20)

tree:  [(function () {return arguments[arguments.length - 1];}), [(function (name, value) {return scm.variable[name] = value;}), "\"x\"", "10"], [(function (l, r) {return l - r;}), [(function (l, r) {return l * r;}), "x", "10"], "20"]]

result:  80

e.g http://tk-scheme.appjet.net/?code=(- 20 10)