v1.7.0: Previous commit did not compile

Amazingly enough, adding an `asSet` property is a breaking change,
because (for example) previously `BTree<number, string>` was implicitly
convertible to `BTree<number, string|number>`, but the new property
somehow causes TypeScript to disallow the conversion while complaining
that "Types of property 'diffAgainst' are incompatible." Yeah...
sometimes I think TypeScript has a lot of bugs.

Author: qwertie

b+tree.d.ts

@@ -1,4 +1,4 @@
-import { ISortedMap, ISortedMapF } from './interfaces';
+import { ISortedMap, ISortedMapF, ISortedSet } from './interfaces';
 export { ISetSource, ISetSink, ISet, ISetF, ISortedSetSource, ISortedSet, ISortedSetF, IMapSource, IMapSink, IMap, IMapF, ISortedMapSource, ISortedMap, ISortedMapF } from './interfaces';
 export declare type EditRangeResult<V, R = number> = {
     value?: V;
@@ -456,5 +456,11 @@ export default class BTree<K = any, V = any> implements ISortedMapF<K, V>, ISort
      *  does check that maxKey() of the children of internal nodes are sorted. */
     checkValid(): void;
 }
+/** A TypeScript helper function that simply returns its argument, typed as
+ *  `ISortedSet<K>` if the BTree implements it, as it does if `V extends undefined`.
+ *  If `V` cannot be `undefined`, it returns `unknown` instead. Or at least, that
+ *  was the intention, but TypeScript is acting weird and may return `ISortedSet<K>`
+ *  even if `V` can't be `undefined` (discussion: btree-typescript issue #14) */
+export declare function asSet<K, V>(btree: BTree<K, V>): undefined extends V ? ISortedSet<K> : unknown;
 /** A BTree frozen in the empty state. */
 export declare const EmptyBTree: BTree<any, any>;

b+tree.js

@@ -15,7 +15,7 @@ var __extends = (this && this.__extends) || (function () {
     };
 })();
 Object.defineProperty(exports, "__esModule", { value: true });
-exports.EmptyBTree = exports.simpleComparator = exports.defaultComparator = void 0;
+exports.EmptyBTree = exports.asSet = exports.simpleComparator = exports.defaultComparator = void 0;
 /**
  * Compares DefaultComparables to form a strict partial ordering.
  *
@@ -1074,11 +1074,20 @@ var BTree = /** @class */ (function () {
     return BTree;
 }());
 exports.default = BTree;
+/** A TypeScript helper function that simply returns its argument, typed as
+ *  `ISortedSet<K>` if the BTree implements it, as it does if `V extends undefined`.
+ *  If `V` cannot be `undefined`, it returns `unknown` instead. Or at least, that
+ *  was the intention, but TypeScript is acting weird and may return `ISortedSet<K>`
+ *  even if `V` can't be `undefined` (discussion: btree-typescript issue #14) */
+function asSet(btree) {
+    return btree;
+}
+exports.asSet = asSet;
 if (Symbol && Symbol.iterator) // iterator is equivalent to entries()
     BTree.prototype[Symbol.iterator] = BTree.prototype.entries;
 BTree.prototype.where = BTree.prototype.filter;
 BTree.prototype.setRange = BTree.prototype.setPairs;
-BTree.prototype.add = BTree.prototype.set;
+BTree.prototype.add = BTree.prototype.set; // for compatibility with ISetSink<K>
 function iterator(next) {
     if (next === void 0) { next = (function () { return ({ done: true, value: undefined }); }); }
     var result = { next: next };

b+tree.ts

@@ -1175,15 +1175,6 @@ export default class BTree<K=any, V=any> implements ISortedMapF<K,V>, ISortedMap
     return this.hasOwnProperty('editRange');
   }
 
-  /** A TypeScript helper function that returns `this as ISortedSet<K>` if this
-   *  BTree implements it, which it does if `V extends undefined`. If `V` cannot 
-   *  be `undefined`, it returns `unknown` instead. Or at least, that was the
-   *  intention, but TypeScript is acting weird and may return `ISortedSet<K>` 
-   *  even if `V` can't be `undefined` (discussion: btree-typescript issue #14) */
-  get asSet(): undefined extends V ? ISortedSet<K> : unknown {
-    return this as any;
-  }
-
   /** Scans the tree for signs of serious bugs (e.g. this.size doesn't match
    *  number of elements, internal nodes not caching max element properly...)
    *  Computational complexity: O(number of nodes), i.e. O(size). This method
@@ -1195,20 +1186,29 @@ export default class BTree<K=any, V=any> implements ISortedMapF<K,V>, ISortedMap
   }
 }
 
+/** A TypeScript helper function that simply returns its argument, typed as 
+ *  `ISortedSet<K>` if the BTree implements it, as it does if `V extends undefined`.
+ *  If `V` cannot be `undefined`, it returns `unknown` instead. Or at least, that
+ *  was the intention, but TypeScript is acting weird and may return `ISortedSet<K>` 
+ *  even if `V` can't be `undefined` (discussion: btree-typescript issue #14) */
+export function asSet<K,V>(btree: BTree<K,V>): undefined extends V ? ISortedSet<K> : unknown {
+  return btree as any;
+}
+
 declare const Symbol: any;
 if (Symbol && Symbol.iterator) // iterator is equivalent to entries()
   (BTree as any).prototype[Symbol.iterator] = BTree.prototype.entries;
 (BTree as any).prototype.where = BTree.prototype.filter;
 (BTree as any).prototype.setRange = BTree.prototype.setPairs;
-(BTree as any).prototype.add = BTree.prototype.set;
+(BTree as any).prototype.add = BTree.prototype.set; // for compatibility with ISetSink<K>
 
 function iterator<T>(next: () => IteratorResult<T> = (() => ({ done:true, value:undefined }))): IterableIterator<T> {
   var result: any = { next };
   if (Symbol && Symbol.iterator)
     result[Symbol.iterator] = function() { return this; };
   return result;
 }
-  
+
 
 /** Leaf node / base class. **************************************************/
 class BNode<K,V> {