From 254dea1bcd1aaf71cc0a87f18e71681b07c0be61 Mon Sep 17 00:00:00 2001
From: Eric Hanson <5846501+ericphanson@users.noreply.github.com>
Date: Tue, 5 Oct 2021 14:46:13 +0200
Subject: [PATCH] Update src/functor.jl

---
 src/functor.jl | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/functor.jl b/src/functor.jl
index e082cd6..312a498 100644
--- a/src/functor.jl
+++ b/src/functor.jl
@@ -193,6 +193,9 @@ julia> fcollect(m, exclude = v -> Functors.isleaf(v))
 ```
 """
 function fcollect(x; output = [], cache = Base.IdSet(), exclude = v -> false)
+    # note: we don't have an `OrderedIdSet`, so we use an `IdSet` for the cache
+    # (to ensure we get exactly 1 copy of each distinct array), and a usual `Vector`
+    # for the results, to preserve traversal order (important downstream!).
     x in cache && return output
     if !exclude(x)
       push!(cache, x)