Macro parallel_internal

macro_rules! parallel_internal {
    (
        // Accumulate a token for each future that has been expanded: "_ _ _".
        current_position: [
            $($underscores:tt)*
        ]
        // Accumulate values and their positions in the tuple: `_0th ()   _1st ( _ ) …`.
        values_and_positions: [
            $($acc:tt)*
        ]
        // Munch one value.
        munching: [
            $current:tt
        ]
    ) => { ... };
    (
        // Accumulate a token for each future that has been expanded: "_ _ _".
        current_position: [
            $($underscores:tt)*
        ]
        // Accumulate values and their positions in the tuple: `_0th ()   _1st ( _ ) …`.
        values_and_positions: [
            $($acc:tt)*
        ]
        // Munch one value.
        munching: [
            $current:tt
            $($rest:tt)+
        ]
    ) => { ... };
    (
        current_position: [
            $($max:tt)*
        ]
        values_and_positions: [
            $(
                $val:tt ( $($pos:tt)* )
            )*
        ]
        munching: []
    ) => { ... };
}