diff --git a/interfaces.d.ts b/interfaces.d.ts
index 9f1e001..e55ee38 100644
--- a/interfaces.d.ts
+++ b/interfaces.d.ts
@@ -84,10 +84,12 @@ export interface ISortedSetSource<K=any> extends ISetSource<K>
   minKey(): K | undefined;
   /** Gets the highest key in the collection. */
   maxKey(): K | undefined;
-  /** Returns the next key larger than the specified key (or undefined if there is none) */
-  nextHigherKey(key: K): K|undefined;
-  /** Returns the next key smaller than the specified key (or undefined if there is none) */
-  nextLowerKey(key: K): K|undefined;
+  /** Returns the next key larger than the specified key (or undefined if there is none).
+   *  Also, nextHigherKey(undefined) returns the lowest key. */
+  nextHigherKey(key?: K): K|undefined;
+  /** Returns the next key smaller than the specified key (or undefined if there is none).
+   *  Also, nextLowerKey(undefined) returns the highest key. */
+  nextLowerKey(key?: K): K|undefined;
   /** Calls `callback` on the specified range of keys, in ascending order by key.
    * @param low The first key scanned will be greater than or equal to `low`.
    * @param high Scanning stops when a key larger than this is reached.
@@ -109,10 +111,12 @@ export interface ISortedSetSource<K=any> extends ISetSource<K>
 /** An data source that provides read-only access to items in sorted order. */
 export interface ISortedMapSource<K=any, V=any> extends IMapSource<K, V>, ISortedSetSource<K>
 {
-  /** Returns the next pair whose key is larger than the specified key (or undefined if there is none) */
-  nextHigherPair(key: K): [K,V]|undefined;
-  /** Returns the next pair whose key is smaller than the specified key (or undefined if there is none) */
-  nextLowerPair(key: K): [K,V]|undefined;
+  /** Returns the next pair whose key is larger than the specified key (or undefined 
+   *  if there is none). If key === undefined, this function returns the lowest pair. */
+  nextHigherPair(key?: K): [K,V]|undefined;
+  /** Returns the next pair whose key is smaller than the specified key (or undefined
+   *  if there is none). If key === undefined, this function returns the highest pair. */
+  nextLowerPair(key?: K): [K,V]|undefined;
   /** Builds an array of pairs from the specified range of keys, sorted by key.
    * Each returned pair is also an array: pair[0] is the key, pair[1] is the value.
    * @param low The first key in the array will be greater than or equal to `low`.
diff --git a/package.json b/package.json
index ce50153..c5c6845 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "sorted-btree",
-  "version": "1.7.0",
+  "version": "1.8.0",
   "description": "A sorted list of key-value pairs in a fast, typed in-memory B+ tree with a powerful API.",
   "main": "b+tree.js",
   "typings": "b+tree",
diff --git a/readme.md b/readme.md
index dc0971a..841ce6c 100644
--- a/readme.md
+++ b/readme.md
@@ -9,10 +9,10 @@ This is a fast B+ tree implementation, largely compatible with the standard Map,
 
 Use `npm install sorted-btree` in a terminal to install it in your npm-based project.
 
-Ukraine
--------
+Ukraine is still under attack
+-----------------------------
 
-I'm writing this on the one-month anniversary of the full-scale invasion of Ukraine.
+I'm writing this on March 24, 2022: the one-month anniversary of the full-scale invasion of Ukraine.
 
 ![Mariupol](http://david.loyc.net/misc/ukraine/Mariupol-from-above.webp)
 
@@ -28,7 +28,7 @@ Without electricity, reports from Mariupol have been limited, but certainly ther
 
 ![Mariupol apartment bombed](http://david.loyc.net/misc/ukraine/Mariupol-explosion.webp)
 
-Here you can see the famous theatre labeled "Children" in huge letters, which Russia bombed anyway.
+Here you can see the famous Donetsk Academic Regional Drama Theatre, labeled "дети" ("children") in huge letters, which held over 1,000 civilians. Russia bombed it anyway.
 
 ![Mariupol theatre](http://david.loyc.net/misc/ukraine/Mariupol-theatre-children.webp)
 ![Mariupol theatre before](http://david.loyc.net/misc/ukraine/Mariupol-theatre-children-before.jpg)
@@ -48,7 +48,7 @@ Or in these other places...
 ![Kharkiv](http://david.loyc.net/misc/ukraine/Kharkiv-firefighters-rubble.webp)
 ![Kharkiv](http://david.loyc.net/misc/ukraine/Kharkiv-two-dead.webp)
 
-What's that? You just wanted a B+ tree? Fair enough. I hope it meets your needs.
+What's that? You just wanted a B+ tree? Well, you're in for a treat.
 
 Features
 --------
@@ -418,6 +418,17 @@ Benchmarks (in milliseconds for integer keys/values)
 Version history
 ---------------
 
+### v1.8.0 ###
+
+- Argument of `ISortedSetSource.nextHigherKey(key: K)` changed to `key?: K`
+- Argument of `ISortedSetSource.nextLowerKey(key: K)` changed to `key?: K`
+- Argument of `ISortedMapSource.nextHigherPair(key: K)` changed to `key?: K`
+- Argument of `ISortedMapSource.nextLowerPair(key: K)` changed to `key?: K`
+
+### v1.7.0 ###
+
+- Added `asSet` method, defined as follows: `asSet<K,V>(btree: BTree<K,V>): undefined extends V ? ISortedSet<K> : unknown { return btree as any; }`
+
 ### v1.6.2 ###
 
 - Bug fixes: two rare situations were discovered in which shared nodes could fail to be marked as shared, and as a result, mutations could affect copies that should have been completely separate.