code: def get_child_by_qualifier(self, parent_qualifier):
          
          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]]
docs: :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