1 SUM
  Syntax: SUM id [&idb] xlo xhi
   where: id   is the plot identifier
          idb  is the (optional) secondary identifier
          xlo  is the lower limit of the range to sum
          xhi  is the upper limit of the range to sum

  Sums the contents of a plot over  the range given. The result is printed
  and is also  stored in register  101, which you can  use with the syntax
  `R101'. The number of points summed over is in register 102.

