function area = triangle_area ( xlist, ylist )
%
%  The X and Y coordinates of the vertices of a triangle are given
%  in xlist and ylist.
%
%  This function computes the area of the triangle.
%
  area = 0.5 * abs ( ...
      xlist(1) * ( ylist(2) - ylist(3) ) ...
    + xlist(2) * ( ylist(3) - ylist(1) ) ...
    + xlist(3) * ( ylist(1) - ylist(2) ) );

  return
end