VARIATION SimpleSet
input: {
    "pipeline": [
        {
            "$set": {
                "a": {
                    "$const": "foo"
                }
            }
        },
        {
            "$match": {
                "a": "foo"
            }
        }
    ]
}
output: {
    "$set:0": {
        "scope:0": {
            "exhaustiveScope": "scope:-1",
            "fields": {
                "<missing>:0": {
                    "declaringScope": "scope:0",
                    "dependencies": []
                },
                "a:1": {
                    "declaringScope": "scope:0",
                    "dependencies": []
                }
            }
        },
        "dependencies": []
    },
    "$match:1": {
        "scope:0": "scope:0",
        "dependencies": [
            "a:1"
        ]
    }
}

