YOU RECEIVE an unambiguous SETI signal that is just a string of prime numbers from some 20 lightyears away. It is up to you to create a response such that it only takes a single call and response (40 years) to enter an open exchange of complex ideas, otherwise you will die before knowing the outcome of your life's work.
Assuming they are an organized civilization of individuals and not a cloud of space gas or a hive mind or anything like that, how do you formulate the message you send back to ensure it doesn't take multiple communications to begin exchanging complex information with them?