#  This function computes the product of the elements of an array.  If
#  `x' is a scalar, it is returned unchanged.  If `x' is a vector, the
#  product of all of its elements is returned.  If `x' is a matrix, a
#  vector is returned, each element of which is the product of the
#  elements in the corresponding column of `x'.  If `x' is a table,
#  the product function is applied to each of its members.

#  A product with zero terms is unity, by definition.

product = function (x)
{
  local (c; s; v; i; n; r);

  c = class (x);

  if (c == "vector")
  {
    s = 1;
    for (x in x) { s @= x; }

  elseif  (c == "matrix")

    n = x.nc;
    s = fill (n; 0);
    if (x.nc)
    {
      for (i in seq (n))
      {
        r = 1;
        for (v in x[;i]) { r @= v; }
        s[i] = r;
      }
    }
    s.eid = x.cid;

  elseif (c == "scalar")

    s = x;

  elseif (c == "table")

    for (v in members (x)) { x.(v) = self (x.(v)); }
    s = x;

  else

    message ("Invalid argument type to product.");
    exception ();
  }

  return s;
};