Updating a player's score in high-score-board has an undefined behavior.

[rhwood]

The exercise of updating a player's score has an undefined behavior that is a teachable opportunity. The undefined behavior is what should happen when an attempt is made to update the score of a player who has not been added. There are two possibilities:

• No change should occur (in which case the exercise should test for that and the student should guard against attempting to change a non-exesitant key in the dictionary)
• The new player should be added (in which case the student should use the dictionary's key base subscript with a default value (sorry I couldn't find a better reference)

I can submit a PR for either behavior, but don't know which to prefer teaching.

swift/exercises/concept/high-score-board/.docs/instructions.md

Line 56 in f5e15a3

## 5. Update a player's score

[kotp]

A third option is to add this undefined behavior as a topic for mentoring. So the PR might be an addition to the mentor notes rather than the tests.