function eval(x, env){
    env = env || global_env;
    if (typeof x == 'string')
        return env.find(x.valueOf())[x.valueOf()];
    else if (typeof x == 'number')
        return x;
    else if (x[0] == 'quote')
        return x[1];
    else if (x[0] == 'if'){
        var test = x[1], conseq = x[2], alt = x[3];
        if (eval(test, env))
            return eval(conseq, env);
        else
            return eval(alt, env);
    }
    else if (x[0] == 'set!')
        env.find(x[1])[x[1]] = eval(x[2], env);
    else if (x[0] == 'define')
        env[x[1]] = eval(x[2], env);
    else if (x[0] == 'lambda'){
        var vars = x[1], exp = x[2];
        return function(){
          return eval(exp, Env({parms: vars, args: arguments, outer: env}));
        };
    }
    else if (x[0] == 'begin'){
         var val;
         for (var i = 0; i < x.length; i += 1)
             val = eval(x[i], env);
         return val;
    }
    else{
        var exps = [];
        for (i = 0; i < x.length; i += 1)
            exps[i] = eval(x[i], env);
        var proc = exps.shift();
        return proc.apply(env, exps);
    }
}