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The following is preferable for code maintenance. By defining what constitutes a failure and explicitly testing for it, the behavior is clearly implied and future modifications are more likely to preserve it. If there is a future modification like the one above, it is immediately obvious that the if statement in processRequest does not utilize the specification of validateUser correctly.

errno_tint validateUser(User usr) {
  if(list_contains(validUsers, usr)) {
    return 1;
  }

  return 0;
}

void processRequest(User usr, Request request) {
  if(validateUser(usr) == 0) {
    return "invalid user";
  }
  else {
    serveResults();
  }
}

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