diff --git a/package-lock.json b/package-lock.json
index 7ac4b8e..87f2658 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -22,6 +22,7 @@
         "@babel/core": "^7.13.1",
         "@babel/eslint-parser": "^7.13.4",
         "@babel/plugin-syntax-class-properties": "^7.12.13",
+        "@types/node": "^14.14.31",
         "eslint": "^7.20.0",
         "nyc": "^15.1.0",
         "open": "^7.4.2",
@@ -547,6 +548,12 @@
         "node": ">=8"
       }
     },
+    "node_modules/@types/node": {
+      "version": "14.14.31",
+      "resolved": "https://registry.npmjs.org/@types/node/-/node-14.14.31.tgz",
+      "integrity": "sha512-vFHy/ezP5qI0rFgJ7aQnjDXwAMrG0KqqIH7tQG5PPv3BWBayOPIQNBjVc/P6hhdZfMx51REc6tfDNXHUio893g==",
+      "dev": true
+    },
     "node_modules/acorn": {
       "version": "7.4.1",
       "resolved": "https://registry.npmjs.org/acorn/-/acorn-7.4.1.tgz",
@@ -3382,6 +3389,12 @@
       "integrity": "sha512-ZXRY4jNvVgSVQ8DL3LTcakaAtXwTVUxE81hslsyD2AtoXW/wVob10HkOJ1X/pAlcI7D+2YoZKg5do8G/w6RYgA==",
       "dev": true
     },
+    "@types/node": {
+      "version": "14.14.31",
+      "resolved": "https://registry.npmjs.org/@types/node/-/node-14.14.31.tgz",
+      "integrity": "sha512-vFHy/ezP5qI0rFgJ7aQnjDXwAMrG0KqqIH7tQG5PPv3BWBayOPIQNBjVc/P6hhdZfMx51REc6tfDNXHUio893g==",
+      "dev": true
+    },
     "acorn": {
       "version": "7.4.1",
       "resolved": "https://registry.npmjs.org/acorn/-/acorn-7.4.1.tgz",
diff --git a/package.json b/package.json
index 687ceb7..47f9b34 100644
--- a/package.json
+++ b/package.json
@@ -17,6 +17,7 @@
     "@babel/core": "^7.13.1",
     "@babel/eslint-parser": "^7.13.4",
     "@babel/plugin-syntax-class-properties": "^7.12.13",
+    "@types/node": "^14.14.31",
     "eslint": "^7.20.0",
     "nyc": "^15.1.0",
     "open": "^7.4.2",
diff --git a/path.js b/path.js
index f862f87..a2fe01e 100644
--- a/path.js
+++ b/path.js
@@ -1,13 +1,31 @@
+// @ts-check
+
 'use strict';
 
+/** A path is a story label optionally with a variable number of ids.
+ * The label may be a dotted string containing sub-labels within each dotted level.
+ * Any ids count nodes within the grammar subtree under label.
+ *
+ * @typedef {[string, ...number[]]} Path
+ */
+
 exports.start = start;
 
+/** Creates the Path where all stories start from.
+ *
+ * @returns {Path}
+ */
 function start() {
     return ['start'];
 }
 
 exports.toName = pathToName;
 
+/** Converts a path to a dotted string like "lab.el.1.2.3"
+ *
+ * @param {Path} path
+ * @returns {string}
+ */
 function pathToName(path) {
     var name = path[0];
     var i;
@@ -23,24 +41,39 @@ function pathToName(path) {
 
 exports.next = nextPath;
 
+/** Constructs a sibling path, incrementing the last id from the given path.
+ * If the given path is label-only, simply returns the given path unchanged.
+ *
+ * @param {Path} path
+ * @returns {Path}
+ */
 function nextPath(path) {
-    path = path.slice();
-    path[path.length - 1]++;
+    const [label, ...ids] = path;
+    if (ids.length > 0) {
+        ids[ids.length-1]++;
+        return [label, ...ids];
+    }
     return path;
 }
 
 exports.firstChild = firstChildPath;
 
+/** Constructs a child path by appending a 1 id to a copy of the given path.
+ *
+ * @param {string|Path} path
+ * @returns {Path} -- a copy of path with an added 1 id
+ */
 function firstChildPath(path) {
-    path = path.slice();
-    path.push(1);
-    return path;
+    return typeof path === 'string' ? [path, 1] : [...path, 1];
 }
 
 exports.zerothChild = zerothChildPath;
 
+/** Constructs a child path by appending a 0 id to a copy of the given path.
+ *
+ * @param {string|Path} path
+ * @returns {Path} -- a copy of path with an added 0 id
+ */
 function zerothChildPath(path) {
-    path = path.slice();
-    path.push(0);
-    return path;
+    return typeof path === 'string' ? [path, 0] : [...path, 0];
 }
diff --git a/tsconfig.json b/tsconfig.json
index a70df0d..8b6b014 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -4,6 +4,8 @@
     "noEmit": true,
 
     "module": "CommonJS",
-    "target": "es2020"
+    "target": "es2020",
+
+    "strict": true
   }
 }