This problem about 
moved so quickly that I had no need to 
any of my code down the 
. This seems more like a Day 1 problem than a Day 4, which leaves me queasy about Days 5-12.
I had a small issue with arrays. Somehow after index 100 the value of an uninitialized array is not valid and I got random “data“ errors. after writing `others => False` everything worked as it should. But finding the error took rather long 
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That’s why I use SPARK!
Well, no, that’s not the reason. I use it because I wanted to learn it a bit, but SPARK does help preventing problems with uninitialized values.
For problem 4b, I used a standard (recursive) flooding algorithm, which in principle is faster than iterating until there are no more changes.
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