VARIATION ExhaustiveStage
input: {
    "pipeline": [
        {
            "$replaceRoot": {
                "newRoot": {
                    "$const": {}
                }
            }
        },
        {
            "$set": {
                "b": {
                    "$const": "bar"
                }
            }
        },
        {
            "$match": {
                "a": "baz"
            }
        }
    ]
}
output: {
    "$replaceRoot:0": {
        "scope:0": {
            "exhaustiveScope": "scope:0",
            "fields": {
                "<missing>:0": {
                    "declaringScope": "scope:0",
                    "dependencies": []
                }
            }
        },
        "dependencies": []
    },
    "$set:1": {
        "scope:1": {
            "exhaustiveScope": "scope:0",
            "fields": {
                "<missing>:1": {
                    "declaringScope": "scope:1",
                    "dependencies": []
                },
                "b:2": {
                    "declaringScope": "scope:1",
                    "dependencies": []
                }
            }
        },
        "dependencies": []
    },
    "$match:2": {
        "scope:1": "scope:1",
        "dependencies": [
            "<missing>:0"
        ]
    }
}

