node.Variable = function Variable(name) { this.name = name };
node.Variable.prototype = {
    eval: function (env) {
        var variables, variable;
        for (var i = 0; i < env.frames.length; i++) {
            variables = env.frames[i].variables();

            for (var j = 0; j < variables.length; j++) {
                variable = variables[j];

                if (variable.name === this.name) {
                    if (variable.value.eval) {
                        return variable.value.eval(env);
                    } else { return variable.value }
                }
            }
        }
        throw new(Error)("variable " + this.name + " is undefined");
    }
};