Orphanet classification system can be depicted by oriented graphs as it is a wide list of parent/child relationships.
The igraph package provides useful tools for graphs analysis that we can use for the
study of rare diseases.
The roots are the ORPHAcodes without any parent,
like the heads of classification. The leaves are the ORPHAcodes without any children.
They are usually disorders or subtypes of disorders (see get_classification_level()).
Examples
graph = get_ancestors('303', output='graph')
roots = find_roots(graph)
leaves = find_leaves(graph)