[ swap nested
     over 1 swap
     witheach *
     of
     2 pack ]           is array    (   x [ --> [ )

  [ 0 peek ]            is [layout] (     [ --> [ )

  [ 1 peek size ]       is [size]   (     [ --> n )

 [ swap unpack
   unrot [a]-> peek ]  is [peek]    (   [ a --> x )

  [ swap unpack
    dip [ tuck [a]-> ]
    rot dip
      [ swap poke ]
    swap 2 pack ]      is [poke]    ( x a [ --> a )