Changes in version 2.9 (2022-08-15)                   

  - Attempting to create ontologies containing cycles now fails with an
    error message and get_ontology now has option merge_equivalent_terms
    (TRUE by default) so that terms that are marked equivalent_to other
    terms are merged and properties aggregated (thanks to feedback from
    Vincent Carey).
  - get_ancestors now returns an empty character vector rather than NULL
    when terms argument is empty.

                  Changes in version 2.2 (2016-10-28)                   

  - The *_branches functions have all been renamed to *_descendants and
    their branch_roots argument to roots.
  - The get_descendants function has had two arguments renamed: ancestor
    is now roots and can accept a vector of root terms, returning the
    set of all terms which descend from them; remove_ancestor is now
    exclude_roots, determining whether or not to exclude the given roots
    terms from the result.
  - A check function which checks whether an ontology is valid has been
    added.
  - XML encoded 'OWL' files can now be read in as ontology_indexes.

                         Changes in version 2.1                         

  - A specialisation of print has been added for ontology_indexes.

                  Changes in version 2.0 (2016-08-03)                   

  - get_ontology now only requires a file argument.
  - ontology_index objects now contain an obsolete slot - a logical
    vector.
  - prune_branches, exclude_branches and intersection_with_branches now
    use the same signature and are renamed to reflect the possibilit of
    passing multiple branch roots.
  - get_term_property, get_term_parents, get_term_ancestors and
    get_term_children functions with as_names argument to get
    descriptive names added for convenience.
  - swap_out_alt_ids replaced with force_compatibility which optionally
    throws an error if non-matchable term IDs are passed.
  - GO now included: data(go) now loads an ontology_index of the Gene
    Ontology (2015-11-27).