EIP-7549 Clarify network vs on chain aggregation

specs/_features/eip7549/validator.md

@@ -26,7 +26,24 @@
 
 ##### Attestations
 
-Attestations received from aggregators with disjoint `committee_bits` sets and equal `AttestationData` SHOULD be consolidated into a single `Attestation` object.
+The network attestation aggregates contain only the assigned committee attestations.
+Attestation aggregates received by the block proposer from the committee aggregators with disjoint `committee_bits` sets and equal `AttestationData` SHOULD be consolidated into a single `Attestation` object.
+The proposer should run the following function to construct an on chain final aggregate form a list of network aggregates with equal `AttestationData`:
+
+```python
+def compute_on_chain_aggregate(network_aggregates: List[Attestation]) -> Attestation:
+    aggregates = sorted(network_aggregates, key=lambda a: get_committee_indices(a.committee_bits)[0])
+
+    data = aggregates[0].data
+    aggregation_bits = [a.aggregation_bits[0] for a in aggregates]
+    signature = bls.Aggregate([a.signature for a in aggregates])
+
+    committee_indices = [get_committee_indices(a.committee_bits)[0] for a in aggregates]
+    committee_flags = [(index in committee_indices) for index in range(0, MAX_COMMITTEES_PER_SLOT)]        
+    committee_bits = Bitvector[MAX_COMMITTEES_PER_SLOT](committee_flags)
+
+    return Attestation(aggregation_bits, data, committee_bits, signature)
+```
 
 ### Attesting