This report was generated with PropEr's master branch on 26/8/2020 commit.
PropEr shrinks this example to a list containing two (more than 85% of the cases) or occasionally one or three inner lists, whose elements are in the range [-4,4].
Some actual examples of these lists are shown below:
[[2,0,1,-1,-2]](shrunk from[[-4],[5,0,1,-3,-8]])[[0,1,-1],[-2,-3]](shrunk from[[0,2,-9],[-2,-4],[-9,-2,0]])[[0,-1],[-2,1],[2]](shrunk from[[5,-2],[-5,1],[2]])
PropEr will never increase the length of a list during shrinking. So its only chance to shrink the counterexample to a single inner list is when the counterexample that it finds to violate the property contains an inner list of length 5 or more, as in case 1 above.
How shrinking works in the other two cases can be understood by taking into consideration that positive integers shrink ("decrease") towards 0 and negative integers "grow" towards 0.
For example, in case 2 above, after splitting the counterexample in two parts and concentrating on its first half (i.e., in the list [[0,2,-9],[-2,-4]]), shrinking discovers that the first integer (0) is already shrunk, then tries to decrease 2 towards 0 but that number is already taken so it stops at 1, then -9 grows to -1, -2 cannot grow to -1 or 0 because these numbers are taken so it remains -2 and finally -4 becomes -3.
A similar reasoning explains the last case.
Over 100 runs, PropEr found a counterexample after 7 to 16 tests, and performed between 1 and 9 shrinking attempts.
See the report of the run for more information.