Mention that Exc with equivalent elements is NP-hard (#233)

Author: Morwenn

docs/Measures-of-disorder.md

@@ -209,6 +209,8 @@ $$
 
 Computes the minimum number of exchanges required to sort $X$, which corresponds to $|X|$ minus the number of cycles in the sequence. A cycle corresponds to a number of elements in a sequence that need to be rotated to be in their sorted position; for example, let $\langle 2, 4, 0, 6, 3, 1, 5 \rangle$ be a sequence, the cycles are $\langle 0, 2 \rangle$ and $\langle 1, 3, 4, 5, 6 \rangle$ so $Exc(X) = |X| - 2 = 5$.
 
+**Warning:** `probe::exc` generally returns a result higher than the minimum number of exchanges required to sort $X$ when it contains *equivalent elements*. This is because extending $Exc$ to *equivalent elements* is a NP-hard problem (see *One the Cost of Interchange Rearrangement in Strings* by Amir et al). The function does handle such elements in some simple cases, but not in the general case.
+
 | Complexity  | Memory      | Iterators     |
 | ----------- | ----------- | ------------- |
 | n log n     | n           | Forward       |