docs: def add(self, intervention, id=None):
          """
          Add an intervention to intervention/human section.
          intervention is either ElementTree or xml snippet
          """
          if self.et is None:
              return
  
          assert isinstance(intervention, six.string_types)
          et = ElementTree.fromstring(intervention)
          component = None
  
          if et.find("ITN") is not None:
              component = ITN(et)
          elif et.find("GVI") is not None:
              component = GVI(et)
          elif et.find("MDA") is not None:
              component = MDA(et)
          elif et.find("TBV") is not None or et.find("PEV") is not None or et.find("BSV") is not None:
              component = Vaccine(et)
          else:
              return
  
          assert isinstance(component.name, six.string_types)
  
          if id is not None:
              assert isinstance(id, six.string_types)
              et.attrib["id"] = id
  
          index = len(self.et.findall("component"))
          self.et.insert(index, et)