{
  "@context": {
    "@vocab": "http://example.org/",
    "input": {"@container": ["@graph", "@index"]}
  },
  "input": {
    "g1": [{"@graph": {"value": "x"}}, {"value": "y"}],
    "g2": [{"@graph": {"value": "a"}}, {"value": "b"}]
  }
}