code: def add(self, intervention, name=None):
          
          if self.et is None:
              return
  
          assert isinstance(intervention, six.string_types)
          et = ElementTree.fromstring(intervention)
          vector_pop = VectorPopIntervention(et)
  
          assert isinstance(vector_pop.name, six.string_types)
  
          if name is not None:
              assert isinstance(name, six.string_types)
              et.attrib["name"] = name
  
          index = len(self.et.findall("intervention"))
          self.et.insert(index, et)
docs: Add an intervention to vectorPop section.
          intervention is either ElementTree or xml snippet