EIP-6914: Update the equivocating indices in the fork-choice store

specs/_features/eip6914/beacon-chain.md

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-EIP-6914 -- The Beacon Chain
+# EIP-6914 -- The Beacon Chain
 
 ## Table of contents
 

specs/_features/eip6914/fork-choice.md

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+# EIP-6914 -- Fork Choice
+
+## Table of contents
+
+<!-- TOC -->
+<!-- START doctoc generated TOC please keep comment here to allow auto update -->
+<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+
+- [Introduction](#introduction)
+- [Fork choice](#fork-choice)
+  - [Handlers](#handlers)
+    - [`on_reused_index`](#on_reused_index)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+<!-- /TOC -->
+
+## Introduction
+
+This is the modification of the fork choice according to EIP-6914.
+
+## Fork choice
+
+A new handler is added with this upgrade:
+
+- `on_reused_index(store, index)` whenever a validator index `index: ValidatorIndex` is reused. That is, [`get_index_for_new_validator()`](./beacon-chain.md#get_index_for_new_validator) provides an index due to a return value of `True` from [`is_reusable_validator()`](./beacon-chain.md#is_reusable_validator).
+
+This new handler is used to update the list of equivocating indices to be synchronized with the canonical chain.
+
+### Handlers
+
+#### `on_reused_index`
+
+```python
+def on_reused_index(store: Store, index: ValidatorIndex) -> None:
+    store.equivocating_indices.discard(index)
+```