protected Object computeAssign(Environment env, Object rvalue) {
ASTree l = left();
if (l instanceof Name) {
env.put(((Name) l).name(), rvalue);
return rvalue;
} else throw new StoneException("bad assignment", this);
}
public Object eval(Environment env) {
Object value = env.get(name());
if (value == null) throw new StoneException("undefined name: " + name(), this);
else return value;
}