print the closure definition/source in Groovy

Anyone who knows how the print the source of a closure in Groovy?

For example, I have this closure (binded to a)

def a = { it.twice() } 

I would like to have the String "it.twice()" or "{ it.twice() }"

Just a simple toString ofcourse won't work:

a.toString(); //results in: Script1$_run_closure1_closure4_closure6@12f1bf0