def iter_hierarchy(self, ontology, size=None, sleep=None):
for term in self.iter_terms(ontology=ontology, size=size, sleep=sleep):<EOL><INDENT>try:<EOL><INDENT>hierarchy_children_link = term['<STR_LIT>'][HIERARCHICAL_CHILDREN]['<STR_LIT>']<EOL><DEDENT>except KeyError:  <EOL><INDENT>continue<EOL><DEDENT>response = requests.get(hierarchy_children_link).json()<EOL>for child_term in response['<STR_LIT>']['<STR_LIT>']:<EOL><INDENT>yield term['<STR_LIT:label>'], child_term['<STR_LIT:label>']<EOL><DEDENT><DEDENT>
Iterates over parent-child relations

        :param str ontology: The name of the ontology
        :param int size: The size of each page. Defaults to 500, which is the maximum allowed by the EBI.
        :param int sleep: The amount of time to sleep between pages. Defaults to 0 seconds.
        :rtype: iter[tuple[str,str]]