docs: def get_output_ref(self, field, index=None, metadata=None):
          '''
          :param field: Output field name of this analysis
          :type field: string
          :param index: If the referenced field is an array, optionally specify an index (starting from 0) to indicate a particular member of the array
          :type index: int
          :param metadata: If the referenced field is of a data object class, a string indicating the metadata that should be read, e.g. "name", "properties.propkey", "details.refgenome"
          :type metadata: string
  
          Returns a dict containing a valid reference to an output of this analysis.
          '''
  
          link = {"$dnanexus_link": {"analysis": self._dxid, "field": field}}
          if index is not None:
              link["$dnanexus_link"]["index"] = index
          if metadata is not None:
              link["$dnanexus_link"]["metadata"] = metadata
          return link