Function sp_state_machine::prune_changes_tries

pub fn prune_changes_tries<H: Hasher, Number: BlockNumber, F: FnMut(H::Out)>(
    storage: &dyn Storage<H, Number>, 
    first: Number, 
    last: Number, 
    current_block: &AnchorBlockId<H::Out, Number>, 
    remove_trie_node: F
) where
    H::Out: Codec, 

Prune obsolete changes tries. Pruning happens at the same block, where highest level digest is created. Pruning guarantees to save changes tries for last min_blocks_to_keep blocks. We only prune changes tries at max_digest_interval ranges.