diff --git a/src/walkmap/routing.clj b/src/walkmap/routing.clj
new file mode 100644
index 0000000..b925341
--- /dev/null
+++ b/src/walkmap/routing.clj
@@ -0,0 +1,18 @@
+(ns walkmap.core
+  "Finding optimal routes to traverse a map."
+    (:require [walkmap.path :as p]
+            [walkmap.polygon :as q]
+            [walkmap.stl :as s]
+            [walkmap.utils :as u]
+            [walkmap.vertex :as v]))
+
+;; Breadth first search is a good algorithm for terrain in which all steps have
+;; equal, but in our world (like the real world), they don't.
+
+;; Reading list:
+;;
+;; https://en.wikipedia.org/wiki/A*_search_algorithm
+;; https://www.redblobgames.com/pathfinding/a-star/introduction.html
+;; https://faculty.nps.edu/ncrowe/opmpaper2.htm
+;;
+;; See https://simon-brooke.github.io/the-great-game/codox/Pathmaking.html
diff --git a/src/walkmap/tag.clj b/src/walkmap/tag.clj
new file mode 100644
index 0000000..aacab67
--- /dev/null
+++ b/src/walkmap/tag.clj
@@ -0,0 +1,50 @@
+(ns walkmap.tag
+  "Code for tagging, untagging, and finding tags on objects. Note the use of
+  the namespaced keyword, `:walkmap.tag/tags`, denoted in this file `::tags`.
+  This is in an attempt to avoid name clashes with other uses of this key."
+  (:require [clojure.set :refer [difference union]]))
+
+(defn tagged?
+  "True if this `object` is tagged with each of these `tags`."
+  [object & tags]
+  (if
+    (map? object)
+    (if
+      (every? keyword? tags)
+      (let [ot (::tags object)]
+        (and
+          (set? ot)
+          (every? ot tags)
+          true))
+      (throw (IllegalArgumentException.
+               (str "Must be keyword(s): " (map type tags)))))
+    (throw (IllegalArgumentException.
+             (str "Must be a map: " (type object))))))
+
+(defn tag
+  "Return an object like this `object` but with these `tags` added to its tags,
+  if they are not already present."
+  [object & tags]
+  (if
+    (map? object)
+    (if
+      (every? keyword? tags)
+      (assoc object ::tags (union (set tags) (::tags object)))
+      (throw (IllegalArgumentException.
+               (str "Must be keyword(s): " (map type tags)))))
+    (throw (IllegalArgumentException.
+             (str "Must be a map: " (type object))))))
+
+(defn untag
+  "Return an object like this `object` but with these `tags` removed from its
+  tags, if present."
+  [object & tags]
+  (if
+    (map? object)
+    (if
+      (every? keyword? tags)
+      (assoc object ::tags (difference (::tags object) (set tags)))
+      (throw (IllegalArgumentException.
+               (str "Must be keywords: " (map type tags)))))
+    (throw (IllegalArgumentException.
+             (str "Must be a map: " (type object))))))
diff --git a/test/walkmap/tag_test.clj b/test/walkmap/tag_test.clj
new file mode 100644
index 0000000..9f0a269
--- /dev/null
+++ b/test/walkmap/tag_test.clj
@@ -0,0 +1,45 @@
+(ns walkmap.tag-test
+  (:require [clojure.test :refer :all]
+            [walkmap.tag :refer :all]))
+
+(deftest tag-tests
+  (testing "Tagging"
+    (is (set? (:walkmap.tag/tags (tag {} :foo :bar :ban :froboz)))
+        "The value of `:walkmap.tag/tags should be a set.")
+    (is (= (count (:walkmap.tag/tags (tag {} :foo :bar :ban :froboz))) 4)
+        "All the tags passed should be added.")
+    (is (:walkmap.tag/tags (tag {} :foo :bar :ban :froboz) :ban)
+        "`:ban` should be present in the set, and, as it is a set, it
+        should be valid to apply it to a keyword.")
+    (is (not ((:walkmap.tag/tags (tag {} :foo :bar :ban :froboz)) :cornflakes))
+        "`:cornflakes should not be present.")
+    (is (true? (tagged? (tag {} :foo :bar :ban :froboz) :bar))
+        "`tagged?` should return an explicit `true`, not any other value.")
+    (is (tagged? (tag {} :foo :bar :ban :froboz) :bar :froboz)
+        "We should be able to test for the presence of more than one tag")
+    (is (= (tagged? (tag {} :foo :bar :ban :froboz) :bar :cornflakes) false)
+        "If any of the queried tags is missing, false should be returned")
+    (is (tagged? (tag (tag {} :foo) :bar) :foo :bar)
+        "We should be able to add tags to an already tagged object")
+    (is (false? (tagged? (tag {} :foo :bar) :cornflakes))
+        "`tagged?` should return an explicit `false` if a queried tag is missing.")
+    (let [object (tag {} :foo :bar :ban :froboz)]
+      (is (= (untag object :cornflakes) object)
+          "Removing a missing tag should have no effect.")
+      (is (tagged? (untag object :foo) :bar :ban :froboz)
+          "All tags not explicitly removed should still be present.")
+      (is (false? (tagged? (untag object :bar) :bar))
+          "But the tag which has been removed should be removed."))
+    (is (thrown? IllegalArgumentException (tag [] :foo))
+        "An exception should be thrown if `object` is not a map: `tag`.")
+    (is (thrown? IllegalArgumentException (tagged? [] :foo))
+        "An exception should be thrown if `object` is not a map: `tagged?`.")
+    (is (thrown? IllegalArgumentException (untag [] :foo))
+        "An exception should be thrown if `object` is not a map: `untag`.")
+    (is (thrown? IllegalArgumentException (tag {} :foo "bar" :ban))
+        "An exception should be thrown if any of `tags` is not a keyword: `tag`.")
+    (is (thrown? IllegalArgumentException (tagged? {} :foo "bar" :ban))
+        "An exception should be thrown if any of `tags` is not a keyword: `tagged?`.")
+    (is (thrown? IllegalArgumentException (untag {} :foo "bar" :ban))
+        "An exception should be thrown if any of `tags` is not a keywordp: `untag`.")))
+