fix for feedback.

https://class.coursera.org/progfun-002/assignment/part_results?part_id=6
Your solution achieved a testing score of 220 out of 240.
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] exists: given {1,2,3,4}
[Observed Error] org.scalatest.exceptions.TestFailedException: 2 should
exist in the given set.
  [exception was thrown] detailed error message in debug output section
below
[Lost Points] 10

[Test Description] exists & filter: even and 3
[Observed Error] org.scalatest.exceptions.TestFailedException: The set
of all even numbers and 3 should contain an odd element, namely 3.
  [exception was thrown] detailed error message in debug output section
below
[Lost Points] 10