diff --git a/src/cubing/alg/parseAlg.ts b/src/cubing/alg/parseAlg.ts
index a931b4dd6..3a8aeed78 100644
--- a/src/cubing/alg/parseAlg.ts
+++ b/src/cubing/alg/parseAlg.ts
@@ -1,6 +1,4 @@
 import { Alg } from "./Alg";
-import { AlgBuilder } from "./AlgBuilder";
-import { algDebugGlobals } from "./debug";
 import type { AlgNode } from "./alg-nodes";
 import { Commutator } from "./alg-nodes/containers/Commutator";
 import { Conjugate } from "./alg-nodes/containers/Conjugate";
@@ -9,6 +7,8 @@ import { LineComment } from "./alg-nodes/leaves/LineComment";
 import { Move, QuantumMove } from "./alg-nodes/leaves/Move";
 import { Newline } from "./alg-nodes/leaves/Newline";
 import { Pause } from "./alg-nodes/leaves/Pause";
+import { AlgBuilder } from "./AlgBuilder";
+import { algDebugGlobals } from "./debug";
 
 type StoppingChar = "," | ":" | "]" | ")";
 
@@ -35,9 +35,12 @@ export function parseQuantumMove(s: string): QuantumMove {
   return new AlgParser().parseQuantumMove(s);
 }
 
+export const startCharIndexKey = Symbol("startCharIndex");
+export const endCharIndexKey = Symbol("endCharIndex");
+
 export interface ParserIndexed {
-  startCharIndex: number;
-  endCharIndex: number;
+  [startCharIndexKey]: number;
+  [endCharIndexKey]: number;
 }
 
 export type Parsed<T extends Alg | AlgNode> = T & ParserIndexed;
@@ -49,17 +52,19 @@ function addCharIndices<T extends Alg | AlgNode>(
   endCharIndex: number,
 ): Parsed<T> {
   const parsedT = t as ParserIndexed & T;
-  parsedT.startCharIndex = startCharIndex;
-  parsedT.endCharIndex = endCharIndex;
+  parsedT[startCharIndexKey] = startCharIndex;
+  parsedT[endCharIndexKey] = endCharIndex;
   return parsedT;
 }
 
 export function transferCharIndex<T extends Alg | AlgNode>(from: T, to: T): T {
-  if ("startCharIndex" in from) {
-    (to as Parsed<T>).startCharIndex = (from as Parsed<T>).startCharIndex;
+  if (startCharIndexKey in from) {
+    (to as Parsed<T>)[startCharIndexKey] = (from as Parsed<T>)[
+      startCharIndexKey
+    ];
   }
-  if ("endCharIndex" in from) {
-    (to as Parsed<T>).endCharIndex = (from as Parsed<T>).endCharIndex;
+  if (endCharIndexKey in from) {
+    (to as Parsed<T>)[endCharIndexKey] = (from as Parsed<T>)[endCharIndexKey];
   }
   return to;
 }
@@ -84,7 +89,10 @@ class AlgParser {
       }
     }
     const newAlg = new Alg(algNodes) as Parsed<Alg>;
-    const { startCharIndex, endCharIndex } = alg;
+    const {
+      [startCharIndexKey]: startCharIndex,
+      [endCharIndexKey]: endCharIndex,
+    } = alg;
     addCharIndices(newAlg, startCharIndex, endCharIndex);
     return newAlg;
   }
diff --git a/src/cubing/twisty/views/TwistyAlgEditor/TwistyAlgEditor.ts b/src/cubing/twisty/views/TwistyAlgEditor/TwistyAlgEditor.ts
index 281048032..ce1399615 100644
--- a/src/cubing/twisty/views/TwistyAlgEditor/TwistyAlgEditor.ts
+++ b/src/cubing/twisty/views/TwistyAlgEditor/TwistyAlgEditor.ts
@@ -13,7 +13,11 @@
 
 import type { ExperimentalParsed } from "../../../alg";
 import { Alg, type Move, type Pause } from "../../../alg";
-import type { Parsed } from "../../../alg/parseAlg";
+import {
+  endCharIndexKey,
+  startCharIndexKey,
+  type Parsed,
+} from "../../../alg/parseAlg";
 import type {
   AlgProp,
   AlgWithIssues,
@@ -219,14 +223,14 @@ export class TwistyAlgEditor extends ManagedCustomElement {
     this.#highlightedLeaf = leaf;
     this.#carbonCopyPrefix.textContent = this.#textarea.value.slice(
       0,
-      leaf.startCharIndex,
+      leaf[startCharIndexKey],
     );
     this.#carbonCopyHighlight.textContent = this.#textarea.value.slice(
-      leaf.startCharIndex,
-      leaf.endCharIndex,
+      leaf[startCharIndexKey],
+      leaf[endCharIndexKey],
     );
     this.#carbonCopySuffix.textContent = this.#padSuffix(
-      this.#textarea.value.slice(leaf.endCharIndex),
+      this.#textarea.value.slice(leaf[endCharIndexKey]),
     );
     this.#carbonCopyHighlight.hidden = false;
   }
diff --git a/src/cubing/twisty/views/TwistyAlgEditor/model.ts b/src/cubing/twisty/views/TwistyAlgEditor/model.ts
index 154ed6ed4..c0c6b8b90 100644
--- a/src/cubing/twisty/views/TwistyAlgEditor/model.ts
+++ b/src/cubing/twisty/views/TwistyAlgEditor/model.ts
@@ -1,7 +1,11 @@
 // TODO: Move this?
 
 import type { Alg } from "../../../alg";
-import type { Parsed } from "../../../alg/parseAlg";
+import {
+  endCharIndexKey,
+  type Parsed,
+  startCharIndexKey,
+} from "../../../alg/parseAlg";
 import {
   type AlgWithIssues,
   algWithIssuesFromString,
@@ -122,13 +126,13 @@ class LeafToHighlightProp extends TwistyPropDerived<
         return null;
       }
       let where: HighlightWhere;
-      if (inputs.targetChar < leafInfo.leaf.startCharIndex) {
+      if (inputs.targetChar < leafInfo.leaf[startCharIndexKey]) {
         where = "before";
-      } else if (inputs.targetChar === leafInfo.leaf.startCharIndex) {
+      } else if (inputs.targetChar === leafInfo.leaf[startCharIndexKey]) {
         where = "start";
-      } else if (inputs.targetChar < leafInfo.leaf.endCharIndex) {
+      } else if (inputs.targetChar < leafInfo.leaf[endCharIndexKey]) {
         where = "inside";
-      } else if (inputs.targetChar === leafInfo.leaf.endCharIndex) {
+      } else if (inputs.targetChar === leafInfo.leaf[endCharIndexKey]) {
         where = "end";
       } else {
         where = "after";
@@ -143,12 +147,12 @@ class LeafToHighlightProp extends TwistyPropDerived<
     // TODO: binary search
     for (const leafInfo of inputs.leafTokens) {
       if (
-        inputs.targetChar < leafInfo.leaf.startCharIndex &&
+        inputs.targetChar < leafInfo.leaf[startCharIndexKey] &&
         lastLeafInfo !== null
       ) {
         return withWhere(lastLeafInfo);
       }
-      if (inputs.targetChar <= leafInfo.leaf.endCharIndex) {
+      if (inputs.targetChar <= leafInfo.leaf[endCharIndexKey]) {
         return withWhere(leafInfo);
       }
       lastLeafInfo = leafInfo;
diff --git a/src/cubing/twisty/views/TwistyAlgViewer.ts b/src/cubing/twisty/views/TwistyAlgViewer.ts
index 5ee8439b0..07071dcd5 100644
--- a/src/cubing/twisty/views/TwistyAlgViewer.ts
+++ b/src/cubing/twisty/views/TwistyAlgViewer.ts
@@ -15,7 +15,7 @@ import {
   ExperimentalIterationDirection,
   experimentalDirect,
 } from "../../alg/cubing-private";
-import type { Parsed } from "../../alg/parseAlg";
+import { startCharIndexKey, type Parsed } from "../../alg/parseAlg";
 import type { MillisecondTimestamp } from "../controllers/AnimationTypes";
 import type { CurrentMoveInfo } from "../controllers/indexer/AlgIndexer";
 import type { AlgWithIssues } from "../model/props/puzzle/state/AlgProp";
@@ -255,7 +255,7 @@ class AlgToDOMTree extends TraversalDownUp<DataDown, DataUp, DataUp> {
       true,
     );
     dataDown.twistyAlgViewer.highlighter.addMove(
-      (move as Parsed<Move>).startCharIndex,
+      (move as Parsed<Move>)[startCharIndexKey],
       element,
     );
     return {
@@ -387,7 +387,7 @@ class MoveHighlighter {
 
   set(move: Parsed<Move> | null): void {
     const newElem = move
-      ? (this.moveCharIndexMap.get(move.startCharIndex) ?? null)
+      ? (this.moveCharIndexMap.get(move[startCharIndexKey]) ?? null)
       : null;
     if (this.currentElem === newElem) {
       return;
@@ -455,7 +455,7 @@ export class TwistyAlgViewer extends HTMLElementShim {
       .alg;
     // TODO: Use proper architecture instead of a heuristic to ensure we have a parsed alg annotated with char indices.
     const parsedAlg =
-      "startCharIndex" in (sourceAlg as Partial<Parsed<Alg>>)
+      startCharIndexKey in (sourceAlg as Partial<Parsed<Alg>>)
         ? sourceAlg
         : Alg.fromString(sourceAlg.toString());
     this.setAlg(parsedAlg);
diff --git a/src/sites/experiments.cubing.net/cubing.js/alg/inspector.ts b/src/sites/experiments.cubing.net/cubing.js/alg/inspector.ts
index fcfafd6f0..577b06fae 100644
--- a/src/sites/experiments.cubing.net/cubing.js/alg/inspector.ts
+++ b/src/sites/experiments.cubing.net/cubing.js/alg/inspector.ts
@@ -1,5 +1,9 @@
 import { Alg, type AlgNode } from "../../../../cubing/alg";
-import type { Parsed } from "../../../../cubing/alg/parseAlg";
+import {
+  endCharIndexKey,
+  startCharIndexKey,
+  type Parsed,
+} from "../../../../cubing/alg/parseAlg";
 import { extract } from "./extractor";
 
 const algElem = document.querySelector("#alg") as HTMLTextAreaElement;
@@ -22,15 +26,18 @@ function updateInspector(s: string): void {
       const parsed = v as Parsed<Alg | AlgNode>;
       inspectorElem.textContent += "\n";
       inspectorElem.textContent += `${name}: `.padStart(12, " ");
-      inspectorElem.textContent += "".padStart(parsed.startCharIndex, " ");
+      inspectorElem.textContent += "".padStart(parsed[startCharIndexKey], " ");
       inspectorElem.textContent += singleLineS.slice(
-        parsed.startCharIndex,
-        parsed.endCharIndex,
+        parsed[startCharIndexKey],
+        parsed[endCharIndexKey],
       );
       inspectorElem.textContent += "\n";
-      inspectorElem.textContent += "".padEnd(12 + parsed.startCharIndex, " ");
+      inspectorElem.textContent += "".padEnd(
+        12 + parsed[startCharIndexKey],
+        " ",
+      );
       inspectorElem.textContent += bracket(
-        parsed.endCharIndex - parsed.startCharIndex,
+        parsed[endCharIndexKey] - parsed[startCharIndexKey],
       );
     }
   } catch (e) {