Nested macros in Sweet.js

Question

How can I do nested macros using sweet JS?

macro a {
    rule { { $e:expr } } => { }
}

macro b {
    rule { test } => { }
}

In this case, b isn't an expression. Can I set that $e can also be a b macro, being possible to do a b test instead of a, b test, that i s valid?