Specify lexicographic ordering of crosslink_data_root

Fix #542.

specs/core/0_beacon-chain.md

@@ -1805,7 +1805,7 @@ For every `slot in range(get_epoch_start_slot(previous_epoch), get_epoch_start_s
 
 * Let `crosslink_data_root` be `state.latest_crosslinks[shard].crosslink_data_root`
 * Let `attesting_validator_indices(crosslink_committee, crosslink_data_root)` be the union of the [validator](#dfn-validator) index sets given by `[get_attestation_participants(state, a.data, a.aggregation_bitfield) for a in current_epoch_attestations + previous_epoch_attestations if a.data.shard == shard and a.data.crosslink_data_root == crosslink_data_root]`.
-* Let `winning_root(crosslink_committee)` be equal to the value of `crosslink_data_root` such that `get_total_balance(state, attesting_validator_indices(crosslink_committee, crosslink_data_root))` is maximized (ties broken by favoring lower `crosslink_data_root` values).
+* Let `winning_root(crosslink_committee)` be equal to the value of `crosslink_data_root` such that `get_total_balance(state, attesting_validator_indices(crosslink_committee, crosslink_data_root))` is maximized (ties broken by favoring lexicographically smallest `crosslink_data_root`).
 * Let `attesting_validators(crosslink_committee)` be equal to `attesting_validator_indices(crosslink_committee, winning_root(crosslink_committee))` for convenience.
 * Let `total_attesting_balance(crosslink_committee) = get_total_balance(state, attesting_validators(crosslink_committee))`.