Update the equivocating indices in the store

Since we sometimes reuse the slashed validator index, we need to remove
it from the list of equivocating indices in the fork-choice store.

specs/_features/eip6914/beacon-chain.md

@@ -1,4 +1,4 @@
-EIP-6914 -- The Beacon Chain
+# EIP-6914 -- The Beacon Chain
 
 ## Table of contents
 

specs/_features/eip6914/fork-choice.md

@@ -0,0 +1,36 @@
+# 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
+
+This new 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)
+```