at is not guaranteed to lie on a regular grid.
    </p>

    <p>
      This program computes a Delaunay triangulation of the data points,
      and then constructs an interpolant triangle by triangle.  Over a given
      triangle, the interpolant is the linear function which matches the
      data already given at the vertices of the triangle.
    </p>

    <p>
      <b>PWL_INTERP_2D</b> requires the R8LIB library.  The test code
      requires the TEST_INTERP_2D library.
    </p>

    <h3 align = "center">