add EIP-4788 feature

specs/_features/eip4788/beacon-chain.md

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+# EIP-4788 -- The Beacon Chain
+
+## 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)
+- [Containers](#containers)
+  - [Extended Containers](#extended-containers)
+    - [`ExecutionPayload`](#executionpayload)
+    - [`ExecutionPayloadHeader`](#executionpayloadheader)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+<!-- /TOC -->
+
+## Introduction
+
+TODO
+
+## Containers
+
+### Extended Containers
+
+#### `ExecutionPayload`
+
+```python
+class ExecutionPayload(Container):
+    # Execution block header fields
+    parent_hash: Hash32
+    fee_recipient: ExecutionAddress  # 'beneficiary' in the yellow paper
+    state_root: Bytes32
+    receipts_root: Bytes32
+    logs_bloom: ByteVector[BYTES_PER_LOGS_BLOOM]
+    prev_randao: Bytes32  # 'difficulty' in the yellow paper
+    block_number: uint64  # 'number' in the yellow paper
+    gas_limit: uint64
+    gas_used: uint64
+    timestamp: uint64
+    extra_data: ByteList[MAX_EXTRA_DATA_BYTES]
+    base_fee_per_gas: uint256
+    # Extra payload fields
+    block_hash: Hash32  # Hash of execution block
+    transactions: List[Transaction, MAX_TRANSACTIONS_PER_PAYLOAD]
+    withdrawals: List[Withdrawal, MAX_WITHDRAWALS_PER_PAYLOAD]
+    parent_beacon_block_root: Root # [New in EIP-4788]
+```
+
+#### `ExecutionPayloadHeader`
+
+```python
+class ExecutionPayloadHeader(Container):
+    # Execution block header fields
+    parent_hash: Hash32
+    fee_recipient: ExecutionAddress
+    state_root: Bytes32
+    receipts_root: Bytes32
+    logs_bloom: ByteVector[BYTES_PER_LOGS_BLOOM]
+    prev_randao: Bytes32
+    block_number: uint64
+    gas_limit: uint64
+    gas_used: uint64
+    timestamp: uint64
+    extra_data: ByteList[MAX_EXTRA_DATA_BYTES]
+    base_fee_per_gas: uint256
+    # Extra payload fields
+    block_hash: Hash32  # Hash of execution block
+    transactions_root: Root
+    withdrawals_root: Root
+    parent_beacon_block_root: Root # [New in EIP-4788]
+```

specs/_features/eip4788/validator.md

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+# EIP-4788 -- Honest Validator
+
+**Notice**: This document is a work-in-progress for researchers and implementers.
+
+## 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)
+- [Prerequisites](#prerequisites)
+- [Helpers](#helpers)
+- [Protocols](#protocols)
+  - [`ExecutionEngine`](#executionengine)
+    - [`get_payload`](#get_payload)
+- [Beacon chain responsibilities](#beacon-chain-responsibilities)
+  - [Block proposal](#block-proposal)
+    - [Constructing the `BeaconBlockBody`](#constructing-the-beaconblockbody)
+      - [ExecutionPayload](#executionpayload)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+<!-- /TOC -->
+
+## Introduction
+
+This document represents the changes to be made in the code of an "honest validator" to implement the EIP-4788 feature.
+
+## Prerequisites
+
+This document is an extension of the [Capella -- Honest Validator](../capella/validator.md) guide.
+All behaviors and definitions defined in this document, and documents it extends, carry over unless explicitly noted or overridden.
+
+All terminology, constants, functions, and protocol mechanics defined in the updated Beacon Chain doc of [Capella](../capella/beacon-chain.md) are requisite for this document and used throughout.
+Please see related Beacon Chain doc before continuing and use them as a reference throughout.
+
+## Helpers
+
+## Protocols
+
+### `ExecutionEngine`
+
+#### `get_payload`
+
+`get_payload` returns the upgraded EIP-4788 `ExecutionPayload` type.
+
+## Beacon chain responsibilities
+
+All validator responsibilities remain unchanged other than those noted below.
+
+### Block proposal
+
+#### Constructing the `BeaconBlockBody`
+
+##### ExecutionPayload
+
+`ExecutionPayload`s are constructed as they were in Capella, except that the parent beacon block root is also supplied.
+
+*Note*: In this section, `state` is the state of the slot for the block proposal _without_ the block yet applied.
+That is, `state` is the `previous_state` processed through any empty slots up to the assigned slot using `process_slots(previous_state, slot)`.
+
+*Note*: The only change made to `prepare_execution_payload` is to add the parent beacon block root as an additional
+parameter to the `PayloadAttributes`.
+
+```python
+def prepare_execution_payload(state: BeaconState,
+                              pow_chain: Dict[Hash32, PowBlock],
+                              safe_block_hash: Hash32,
+                              finalized_block_hash: Hash32,
+                              suggested_fee_recipient: ExecutionAddress,
+                              execution_engine: ExecutionEngine) -> Optional[PayloadId]:
+    if not is_merge_transition_complete(state):
+        is_terminal_block_hash_set = TERMINAL_BLOCK_HASH != Hash32()
+        is_activation_epoch_reached = get_current_epoch(state) >= TERMINAL_BLOCK_HASH_ACTIVATION_EPOCH
+        if is_terminal_block_hash_set and not is_activation_epoch_reached:
+            # Terminal block hash is set but activation epoch is not yet reached, no prepare payload call is needed
+            return None
+
+        terminal_pow_block = get_terminal_pow_block(pow_chain)
+        if terminal_pow_block is None:
+            # Pre-merge, no prepare payload call is needed
+            return None
+        # Signify merge via producing on top of the terminal PoW block
+        parent_hash = terminal_pow_block.block_hash
+    else:
+        # Post-merge, normal payload
+        parent_hash = state.latest_execution_payload_header.block_hash
+
+    # Set the forkchoice head and initiate the payload build process
+    payload_attributes = PayloadAttributes(
+        timestamp=compute_timestamp_at_slot(state, state.slot),
+        prev_randao=get_randao_mix(state, get_current_epoch(state)),
+        suggested_fee_recipient=suggested_fee_recipient,
+        withdrawals=get_expected_withdrawals(state),
+        parent_beacon_block_root=hash_tree_root(state.latest_block_header), # [New in EIP-4788]
+    )
+    return execution_engine.notify_forkchoice_updated(
+        head_block_hash=parent_hash,
+        safe_block_hash=safe_block_hash,
+        finalized_block_hash=finalized_block_hash,
+        payload_attributes=payload_attributes,
+    )
+```