{
  "type": "project",
  "input": {
    "type": "join",
    "input": [
      {
        "type": "filter",
        "input": {
          "type": "bgp",
          "patterns": [
            {
              "type": "pattern",
              "termType": "Quad",
              "subject": {
                "termType": "Variable",
                "value": "s"
              },
              "predicate": {
                "termType": "Variable",
                "value": "p"
              },
              "object": {
                "termType": "Variable",
                "value": "o"
              },
              "graph": {
                "termType": "NamedNode",
                "value": "http://graph.example.com/"
              }
            }
          ]
        },
        "expression": {
          "type": "expression",
          "subType": "operator",
          "operator": "uri",
          "args": [
            {
              "type": "expression",
              "subType": "term",
              "term": {
                "termType": "Variable",
                "value": "s"
              }
            }
          ]
        }
      },
      {
        "type": "bgp",
        "patterns": [
          {
            "type": "pattern",
            "termType": "Quad",
            "subject": {
              "termType": "Variable",
              "value": "s"
            },
            "predicate": {
              "termType": "Variable",
              "value": "p"
            },
            "object": {
              "termType": "Variable",
              "value": "o"
            },
            "graph": {
              "termType": "NamedNode",
              "value": "http://other.example.com/"
            }
          }
        ]
      }
    ]
  },
  "variables": [
    {
      "termType": "Variable",
      "value": "s"
    }
  ]
}