How to add an operator `in` ?

[bric3]

I understand that it is not possible to write a planout DSL that activate an experience for a set of users.

e.g., given a user_id, I'd like to write :

 - definition: exp_android_toggle
   assign: !planout |
     beta_tester = ['uuid1','uuid2', etc...];
     if(user_id in beta_testers) {
       toggle = true;
     }