static new Map<S, S> slackGetUserName_map;

static synchronized S slackGetUserName(S token, S userID) {
  if (userID == null) ret null;
  S userName = slackGetUserName_map.get(userID);
  if (userName == null) pcall {
    userName = (S) slackGetUserInfo(token, userID).get("name");
    slackGetUserName_map.put(userID, userName);
  }
  ret userName;
}