code: def parse_derived_variable(self, node):
          
  
          if 'name' in node.lattrib:
              name = node.lattrib['name']
          elif 'exposure' in node.lattrib:
              name = node.lattrib['exposure']
          else:
              self.raise_error('<DerivedVariable> must specify a name')
  
          params = dict()
          for attr_name in ['dimension', 'exposure', 'select', 'value', 'reduce', 'required']:
              if attr_name in node.lattrib:
                  params[attr_name] = node.lattrib[attr_name]
  
          self.current_regime.add_derived_variable(DerivedVariable(name, **params))
docs: Parses <DerivedVariable>
  
          @param node: Node containing the <DerivedVariable> element
          @type node: xml.etree.Element
  
          @raise ParseError: Raised when no name of specified for the derived variable.