*procedure
**
PACK
      To pack the characters in a vector into a string.
      size := pack ( vector, string )
      The zero element of  the  vector  should  contain  the
number of characters to be packed; this is anded with a mask
of 255 ( =#XFF ) to prevent the string being longer than 255
characters.
      The specified numbers of characters  are  then  packed
into the string such that
      vector ! i -> string % i
etc.  The  length  is  inserted, and any unused bytes in the
last word of the string are set to zero. The string and vec-
tor  may  be coincident ( ie., @vector = @string ), but they
may not otherwise overlap.
      The result is the number of words used in the string.


