So, I've got my filter methods working properly, but for some reason, it doesn't seem to work with tweets that have been retweeted more than 321 times (weird). Now, here's the code for filter, which works as it should, now I'm just trying to pinpoint why exactly I'm running errors with tweets that have been "retweeted more than 321 times".

Here's the error messages:

======== LOG OF FAILED TESTS ========

Your solution achieved a testing score of 80 out of 110.
Below you can see a short feedback for every test that failed, indicating the reason
for the test failure and how many points you lost for each individual test.

[Test Description] filter: tweet with 321 retweets
[Observed Error] org.scalatest.exceptions.TestFailedException: exactly one such tweet
  [exception was thrown] detailed error message in debug output section below
[Lost Points] 10

[Test Description] filter and union: tweets with 321 and 205 retweets
[Observed Error] org.scalatest.exceptions.TestFailedException: exactly one such tweet
  [exception was thrown] detailed error message in debug output section below
[Lost Points] 10

[Test Description] filter and trending: tweets with 321 and 205 retweets
[Observed Error] org.scalatest.exceptions.TestFailedException: exactly two such tweets
  [exception was thrown] detailed error message in debug output section below
[Lost Points] 10

Here's the filter and filter0 code:

Under abstract:
     def filter(p: Tweet => Boolean): TweetSet = filter0(p, new Empty)
     def filter0(p: Tweet => Boolean, accu: TweetSet): TweetSet

  Under Empty:
     def filter0(p: Tweet => Boolean, accu: TweetSet): TweetSet = accu

  Under NonEmpty
       def filter0(p: Tweet => Boolean, accu: TweetSet): TweetSet = {
         if (p(elem)) this.tail.filter0(p, accu.incl(elem))
         else this.tail.filter0(p, accu)
       }

For empty:
Since the set is now empty, it returns the accumulator, which is a new set of tweets.

For nonempty:
It checks to see if the predicate is true for this element, if true the method calls itself with
the remaining elements (tweets -> this.tail) in the set and adds this tweet
(includes -> accu.incl(elem)), the current element, to the accumulator.
If the predicate is NOT true for this element, the method calls itself but does NOT add the
element to the accumulator.

Now how come I keep getting an error, specifically, "tweet with 321 retweets"?
I notice that union and trending may have issues as well, but of all the three error messages, they
all have filter in common, so I'm guessing this is the root cause.

Additionally, my trending and union methods seem to work fine. 

I wouldn't focus on the number 321. That's probably just one of the data points they use.

We talked about Elvin's problem earlier today. Can't say much about the second one without knowing more.