Make 'Any active validator' statement more strict.

specs/core/0_beacon-chain.md

@@ -1862,29 +1862,28 @@ Note: When applying penalties in the following balance recalculations implemente
 
 ##### Justification and finalization
 
-Note: Rewards and penalties are for participation in the previous epoch, so the "active validator" set is drawn from `get_active_validator_indices(state.validator_registry, previous_epoch)`.
-
+* Let `previous_active_validator_indices = get_active_validator_indices(state.validator_registry, previous_epoch)`
 * Let `epochs_since_finality = next_epoch - state.finalized_epoch`.
 
 Case 1: `epochs_since_finality <= 4`:
 
 * Expected FFG source:
   * Any [validator](#dfn-validator) `index` in `previous_epoch_attester_indices` gains `base_reward(state, index) * previous_epoch_attesting_balance // previous_total_balance`.
-  * Any [active validator](#dfn-active-validator) `index` not in `previous_epoch_attester_indices` loses `base_reward(state, index)`.
+  * Any [active validator](#dfn-active-validator) `index` from `previous_active_validator_indices` not in `previous_epoch_attester_indices` loses `base_reward(state, index)`.
 * Expected FFG target:
   * Any [validator](#dfn-validator) `index` in `previous_epoch_boundary_attester_indices` gains `base_reward(state, index) * previous_epoch_boundary_attesting_balance // previous_total_balance`.
-  * Any [active validator](#dfn-active-validator) `index` not in `previous_epoch_boundary_attester_indices` loses `base_reward(state, index)`.
+  * Any [active validator](#dfn-active-validator) `index` from `previous_active_validator_indices` not in `previous_epoch_boundary_attester_indices` loses `base_reward(state, index)`.
 * Expected beacon chain head:
   * Any [validator](#dfn-validator) `index` in `previous_epoch_head_attester_indices` gains `base_reward(state, index) * previous_epoch_head_attesting_balance // previous_total_balance)`.
-  * Any [active validator](#dfn-active-validator) `index` not in `previous_epoch_head_attester_indices` loses `base_reward(state, index)`.
+  * Any [active validator](#dfn-active-validator) `index` from `previous_active_validator_indices` not in `previous_epoch_head_attester_indices` loses `base_reward(state, index)`.
 * Inclusion distance:
   * Any [validator](#dfn-validator) `index` in `previous_epoch_attester_indices` gains `base_reward(state, index) * MIN_ATTESTATION_INCLUSION_DELAY // inclusion_distance(state, index)`
 
 Case 2: `epochs_since_finality > 4`:
 
-* Any [active validator](#dfn-active-validator) `index` not in `previous_epoch_attester_indices`, loses `inactivity_penalty(state, index, epochs_since_finality)`.
-* Any [active validator](#dfn-active-validator) `index` not in `previous_epoch_boundary_attester_indices`, loses `inactivity_penalty(state, index, epochs_since_finality)`.
-* Any [active validator](#dfn-active-validator) `index` not in `previous_epoch_head_attester_indices`, loses `base_reward(state, index)`.
+* Any [active validator](#dfn-active-validator) `index` from `previous_active_validator_indices` not in `previous_epoch_attester_indices`, loses `inactivity_penalty(state, index, epochs_since_finality)`.
+* Any [active validator](#dfn-active-validator) `index` from `previous_active_validator_indices` not in `previous_epoch_boundary_attester_indices`, loses `inactivity_penalty(state, index, epochs_since_finality)`.
+* Any [active validator](#dfn-active-validator) `index` from `previous_active_validator_indices` not in `previous_epoch_head_attester_indices`, loses `base_reward(state, index)`.
 * Any [active validator](#dfn-active-validator) `index` with `validator.slashed is True`, loses `2 * inactivity_penalty(state, index, epochs_since_finality) + base_reward(state, index)`.
 * Any [validator](#dfn-validator) `index` in `previous_epoch_attester_indices` loses `base_reward(state, index) - base_reward(state, index) * MIN_ATTESTATION_INCLUSION_DELAY // inclusion_distance(state, index)`