nested named graphs

Hi,

I am getting myself familiar with named graphs and with the possibility to include subgraphs into the data. I saw a couple of examples demonstrating signing chains that include nested graphs like this (in Trig format):

@prefix rdf:     <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix foaf:    <http://xmlns.com/foaf/0.1/> .
@prefix sig:     <http://www.example.com/signature#> .
@prefix :     <http://www.example.com/graph#> .
:G2 {
   :G1 {
   _:bnode1
      rdf:type foaf:Person ;
      foaf:name "Manu Sporny" ;
      foaf:homepage
<http://manu.sporny.org/> .
   }
   :G1 sig:signature [
      rdf:type sig:JsonldSignature ;
      sig:signer <http://manu.sporny.org/webid#key-5> ;
      sig:signatureValue "OGQzNGVkMzVmMmQ3ODIyOWM32MzQzNmExMgoYzI4ZDY3NjI4NTIyZTk=" . ] .

:G2 sig:signature [
   rdf:type sig:JsonldSignature ;
   sig:signer <http://authority.payswarm.com/webid#key-873> ;
   sig:signatureValue "kMzVmMVDIyOWM32MzI4ZDY3NjI4mQ3OOGQzNGNTIyZTkQzNmExMgoYz=" . ] .

  
But I could not read this RDF into Jena Dataset (parser error). Do I understand it correctly that Jena and current
RDF specification don't support such nested graphs? Is there is any known/accepted work around when similar representation of the data as in the example above is desirable for the application?

Thanks,
Yana