code: def link(g: Graph, subject: Node, predicate: URIRef) -> Tuple[Optional[URIRef], Optional[URIRef]]:
      
      link_node = g.value(subject, predicate)
      if link_node:
          l = g.value(link_node, FHIR.link)
          if l:
              typ = g.value(l, RDF.type)
              return l, typ
      return None, None
docs: Return the link URI and link type for subject and predicate
      :param g: graph context
      :param subject: subject of linke
      :param predicate: link predicate
      :return: URI and optional type URI.  URI is None if not a link