 by the
  following example with op(A) = A, op(B) = B, n = 5, k = 2 and
  nb = 2:

       ( a  r  r  a  a  )       ( g  g  g  r  r  g  g  )
       ( a  r  r  a  a  )       ( g  g  g  r  r  g  g  )
       ( r  r  r  r  r  )       ( r  r  r  r  r  r  r  )
   A = ( u2 r  r  r  r  ),  G = ( r  r  r  r  r  r  r  ),
       ( u2 u2 r  a  a  )       ( g  g  g  r  r  g  g  )
       ( u2 u2 r  a  a  )       ( g  g  g  r  r  g  g  )
       ( u2 u2 r  a  a  )       ( g  g  g  r  r  g  g  )

       ( t  t  v1 v1 v1 )   