Macro generics::concat

macro_rules! concat {
    (
        $callback:path { $($callback_args:tt)* }
        $($(
            [$($g:tt)*] [$($r:tt)*] [$($w:tt)*]
        ),+ $(,)?)?
    ) => { ... };
}

Concats several parse calls results together.

This macro accepts an input in the following form:

$callback:path { $($callback_args:tt)* }
$($(
    [$(<$($g:tt)*>)?] [$(<$($r:tt)*>)?] [$(where $($w:tt)*)?]
),+ $(,)?)?

and expands into

$callback! {
    $($callback_args)*
    [$(<$($g)+>)?] [$(<$($r)+>)?] [$(where $($w)+)?]
}