The PACK  procedure  copies  components  of  an  unpacked  array
  variable to a packed array variable.

  Syntax:

     PACK( a,i,z )

  The parameter 'a' is an unpacked array.  The parameter 'i' is  a
  value  to  indicate the starting value of the index of 'a'.  The
  parameter 'z' is a packed array of the same  component  type  as
  'a'.

  The number of components in parameter a must be greater than  or
  equal  to  the  number of components in 'z'.  The PACK procedure
  assigns the components of 'a', starting with a[i], to the  array
  'z',  starting  with z[lower bound], until all the components in
  'z' are filled.

  In general, when specifying 'i', keep in  mind  that  the  upper
  bound of a (that is, n) must be greater than or equal to i + v -
  u, where 'v' is the upper bound of 'z'  and  'u'  is  the  lower
  bound  of 'z'.  That is, ORD(n) must be greater than or equal to
  ORD(i) + ORD(v) - ORD(u).