# This function computes the "difference" of a vector.  If vector `v'
# has `n' elements, then this function returns the vector
#
#     v[2]-v[1], v[3]-v[2], ..., v[n]-v[n-1]
#
# The return vector has one fewer element than `v'.  If `v' has
# labels, then the return vector has the labels `v.eid[seq(n-1)]'.

diff = function (x)
{
  x = vector (x);

  if (x.ne < 2)
  {
    message ("run time error: Fewer than 2 elements for difference.");
    exception ();
  }

  return unlabel (x[2:x.ne]) - x[1:x.ne-1];
};