:param parent_qualifier: time_qualifier of the parent process
        :return: <HierarchyEntry> child entry to the HierarchyEntry associated with the parent_qualifier
                or None if the given parent_qualifier is not registered in this hierarchy
                or None if the given parent_qualifier is the bottom process

def get_child_by_qualifier(self, parent_qualifier):
        """
        :param parent_qualifier: time_qualifier of the parent process
        :return: <HierarchyEntry> child entry to the HierarchyEntry associated with the parent_qualifier
                or None if the given parent_qualifier is not registered in this hierarchy
                or None if the given parent_qualifier is the bottom process
        """
        if parent_qualifier not in self.qualifiers:
            return None

        process_qualifiers = list(self.qualifiers)
        if parent_qualifier == process_qualifiers[-1]:
            return None

        parent_index = process_qualifiers.index(parent_qualifier)
        return self.qualifiers[process_qualifiers[parent_index + 1]]