Macro aerosol::join

macro_rules! join {
    {
        $caller:tt
        sep = [{ $($sep:tt)* }]
    } => { ... };
    {
        $caller:tt
        sep = [{ $($sep:tt)* }]
        item = [{ $($head:tt)* }]
        $(
            item = [{ $($tail:tt)* }]
        )+
    } => { ... };
    {
        $caller:tt
        sep = [{ $($sep:tt)* }]
        item = [{ $($head:tt)* }]
    } => { ... };
    {
        $caller:tt
        prepend = [{ $($prepend:tt)* }]
        joined = [{ $($joined:tt)* }]
    } => { ... };
}