Obtain Xmeta `(...)` content as tokens, ie not insisting on a string literal

I'd like to be able to clone the #[builder(derive(..))] feature, but it doesn't seem to be possible to treat everything under a $tmeta as something to extract.

Here's a thing that doesn't work:

define_derive_deftly! {
    pub Builder =

    #[derive( ${tmeta(builder(derive))} )]
    pub struct $<$tname Builder> {

    }
}

#[derive(Deftly)]
#[derive_deftly(Builder)]
#[deftly(builder(derive(Clone,Debug)))]
pub struct MyStruct {
}

Instead, I've had to say:

define_derive_deftly! {
    pub Builder =

    #[derive( ${tmeta(builder(derive)) as tokens} )]
    pub struct $<$tname Builder> {

    }
}

#[derive(Deftly)]
#[derive_deftly(Builder)]
#[deftly(builder(derive = "Clone, Debug"))]
pub struct MyStruct {
}

Maybe we could have a new $Xmeta_content that expands to everything inside of the metadata node. (That is, for #[deftly(builder(derive(Clone,Debug)))], ${Xmeta_content(builder_derive)} would expand to Clone, Debug.