def powers( m ) = map( (^ m), 0.. )

def
  filtered( s@sh:_, ch:ct ) | sh > ch = filtered( s, ct )
  filtered( sh:st, c@ch:_ ) | sh < ch = sh # filtered( st, c )
  filtered( _:st, c ) = filtered( st, c )

println( filtered(powers(2), powers(3)).drop(20).take(10) )