Function 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.