Letter key holding modifier

{
    "description": "Switch to the app launcher when the u key is held down",
    "manipulators": [
        {
            "type": "basic",
            "from": {
                "key_code": "u",
                "modifiers": { "optional": ["any"] }
            },
            "to_if_held_down": [
                {
                    "set_variable": {
                        "name": "u_flag",
                        "value": true
                    }
                }
            ],
            "to_after_key_up": [
                {
                    "set_variable": {
                        "name": "u_flag",
                        "value": false
                    },
                    // Cancel the `to_delayed_action`
                    "halt": true
                }
            ],
            "to_if_alone": [{ "key_code": "u" }],
            "to_delayed_action": {
                "to_if_canceled": [{ "key_code": "u" }]
            },
            "parameters": {
                "basic.to_delayed_action_delay_milliseconds": 250,
                "basic.to_if_alone_timeout_milliseconds": 250,
                "basic.to_if_held_down_threshold_milliseconds": 250
            }
        },

        //
        // Open Safari
        //

        {
            "type": "basic",
            "from": { "key_code": "s" },
            "conditions": [
                { "name": "u_flag", "type": "variable_if", "value": true }
            ],
            "to": [
                {
                    "software_function": {
                        "open_application": {
                            "bundle_identifier": "com.apple.Safari"
                        }
                    }
                }
            ]
        },

        //
        // Open Terminal
        //

        {
            "type": "basic",
            "from": { "key_code": "t" },
            "conditions": [
                { "name": "u_flag", "type": "variable_if", "value": true }
            ],
            "to": [
                {
                    "software_function": {
                        "open_application": {
                            "bundle_identifier": "com.apple.Terminal"
                        }
                    }
                }
            ]
        }
    ]
}