code: def get_subgroupsiteminsertion(cls, itemgroup, modelname, indent) -> str:
          
          subs = []
          model = importtools.prepare_model(modelname)
          for subvars in cls._get_subvars(model):
              subs.append(cls.get_subgroupiteminsertion(
                      itemgroup, model, subvars, indent))
          return '\n'.join(subs)
docs: Return a string defining the required types for the given
          combination of an exchange item group and an application model.
  
          >>> from hydpy.auxs.xmltools import XSDWriter
          >>> print(XSDWriter.get_subgroupsiteminsertion(
          ...     'setitems', 'hland_v1', 1))    # doctest: +ELLIPSIS
              <element name="control"
                       minOccurs="0"
                       maxOccurs="unbounded">
          ...
              </element>
              <element name="inputs"
          ...
              <element name="fluxes"
          ...
              <element name="states"
          ...
              <element name="logs"
          ...