I need to write a rule to identify a missing element in a group of facts.

For example, let us say I want to ensure that my fence has a fence post
every 8 feet.  My fact declaration is as follows:

declare FencePost
 identifier : String
 distanceFromStart: Long
end

If I add the following facts into my session:

FencePost("a", 0)
FencePost("x", 8)
FencePost("e", 24)

I want a rule that will notify the user that there is a missing fence post
between "x" and "e".

I've been looking into using "collect" into a TreeSet/TreeMap to get a
collection sorted on the "distanceFromStart", but I cannot figure out how to
compare adjacent elements in the collection.

Any ideas?

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