Closure's visibility to itself

Problem: Within a closure, trying to reference a variable that references itself fails. For instance:

def items = []
def walker = { child ->
                items << child
                items << walker
         }

This doesn't work: walker is not found within the closure. Defining and assigning the variable does work:

def items = []
def walker
walker = { child ->
                items << child
                items << walker
         }