Help understanding syntax in S06 "Unpacking tree node parameters"

multi traverse ( NAry $top ( :kids [$eldest, *@siblings] ) ) {

The inner signature is

   :( :kids [$eldest, *@siblings] )

This starts out like a named-only parameter, 
   :xx  or  :xx($yy)
but then there is a space and an array.
   :xx @yy

I don't follow that.

• Help understanding syntax in S06 "Unpacking tree node parameters" by John M. Dlugosz