Add doctest for derangements

Author: FedericoStra

src/permutations.jl

@@ -142,6 +142,18 @@ Generate all derangements of an indexable object `a` in lexicographic order.
 Because the number of derangements can be very large, this function returns an iterator object.
 Use `collect(derangements(a))` to get an array of all derangements.
 Only works for `a` with defined length.
+
+# Examples
+```jldoctest
+julia> derangements("julia") |> collect
+44-element Vector{Vector{Char}}:
+ ['u', 'j', 'i', 'a', 'l']
+ ['u', 'j', 'a', 'l', 'i']
+ ['u', 'l', 'j', 'a', 'i']
+ ['u', 'l', 'i', 'a', 'j']
+ ['u', 'l', 'a', 'j', 'i']
+[...]
+```
 """
 derangements(a) = (d for d in multiset_permutations(a, length(a)) if all(t -> t[1] != t[2], zip(a, d)))