karabiner.json

                        "description": "Change fn + letter to left_control + letter",
                        "manipulators": [
                            {
                                "from": {
                                    "key_code": "tab",
                                    "modifiers": {
                                        "mandatory": [
                                            "fn"
                                        ],
                                        "optional": [
                                            "any"
                                        ]
                                    }
                                },
                                "to": [
                                    {
                                        "key_code": "tab",
                                        "modifiers": [
                                            "left_control"
                                        ]
                                    }
                                ],
                                "type": "basic"
                            },