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diff --git a/third_party/immer/immer/detail/hamts/champ_iterator.hpp b/third_party/immer/immer/detail/hamts/champ_iterator.hpp
new file mode 100644
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+++ b/third_party/immer/immer/detail/hamts/champ_iterator.hpp
@@ -0,0 +1,148 @@
+//
+// immer: immutable data structures for C++
+// Copyright (C) 2016, 2017, 2018 Juan Pedro Bolivar Puente
+//
+// This software is distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE or copy at http://boost.org/LICENSE_1_0.txt
+//
+
+#pragma once
+
+#include <immer/detail/hamts/champ.hpp>
+#include <immer/detail/iterator_facade.hpp>
+
+namespace immer {
+namespace detail {
+namespace hamts {
+
+template <typename T, typename Hash, typename Eq, typename MP, bits_t B>
+struct champ_iterator
+    : iterator_facade<champ_iterator<T, Hash, Eq, MP, B>,
+                      std::forward_iterator_tag,
+                      T,
+                      const T&,
+                      std::ptrdiff_t,
+                      const T*>
+{
+    using tree_t = champ<T, Hash, Eq, MP, B>;
+    using node_t = typename tree_t::node_t;
+
+    struct end_t
+    {};
+
+    champ_iterator() = default;
+
+    champ_iterator(const tree_t& v)
+        : depth_{0}
+    {
+        if (v.root->datamap()) {
+            cur_ = v.root->values();
+            end_ = v.root->values() + popcount(v.root->datamap());
+        } else {
+            cur_ = end_ = nullptr;
+        }
+        path_[0] = &v.root;
+        ensure_valid_();
+    }
+
+    champ_iterator(const tree_t& v, end_t)
+        : cur_{nullptr}
+        , end_{nullptr}
+        , depth_{0}
+    {
+        path_[0] = &v.root;
+    }
+
+    champ_iterator(const champ_iterator& other)
+        : cur_{other.cur_}
+        , end_{other.end_}
+        , depth_{other.depth_}
+    {
+        std::copy(other.path_, other.path_ + depth_ + 1, path_);
+    }
+
+private:
+    friend iterator_core_access;
+
+    T* cur_;
+    T* end_;
+    count_t depth_;
+    node_t* const* path_[max_depth<B> + 1];
+
+    void increment()
+    {
+        ++cur_;
+        ensure_valid_();
+    }
+
+    bool step_down()
+    {
+        if (depth_ < max_depth<B>) {
+            auto parent = *path_[depth_];
+            if (parent->nodemap()) {
+                ++depth_;
+                path_[depth_] = parent->children();
+                auto child    = *path_[depth_];
+                if (depth_ < max_depth<B>) {
+                    if (child->datamap()) {
+                        cur_ = child->values();
+                        end_ = cur_ + popcount(child->datamap());
+                    }
+                } else {
+                    cur_ = child->collisions();
+                    end_ = cur_ + child->collision_count();
+                }
+                return true;
+            }
+        }
+        return false;
+    }
+
+    bool step_right()
+    {
+        while (depth_ > 0) {
+            auto parent = *path_[depth_ - 1];
+            auto last   = parent->children() + popcount(parent->nodemap());
+            auto next   = path_[depth_] + 1;
+            if (next < last) {
+                path_[depth_] = next;
+                auto child    = *path_[depth_];
+                if (depth_ < max_depth<B>) {
+                    if (child->datamap()) {
+                        cur_ = child->values();
+                        end_ = cur_ + popcount(child->datamap());
+                    }
+                } else {
+                    cur_ = child->collisions();
+                    end_ = cur_ + child->collision_count();
+                }
+                return true;
+            }
+            --depth_;
+        }
+        return false;
+    }
+
+    void ensure_valid_()
+    {
+        while (cur_ == end_) {
+            while (step_down())
+                if (cur_ != end_)
+                    return;
+            if (!step_right()) {
+                // end of sequence
+                assert(depth_ == 0);
+                cur_ = end_ = nullptr;
+                return;
+            }
+        }
+    }
+
+    bool equal(const champ_iterator& other) const { return cur_ == other.cur_; }
+
+    const T& dereference() const { return *cur_; }
+};
+
+} // namespace hamts
+} // namespace detail
+} // namespace immer