foo = select(soong_config_variable("my_namespace", "my_var"), {
    //       ^ function.builtin
    "foo": unset,
    //     ^ variable.builtin
    default: true,
    // <- variable.builtin
})

/* Assigning to builtins is conveniently not allowed at runtime */
unset = 12
// <- variable.builtin
default = 27
// <- variable.builtin