double t2_xlist[] = {  0.0,     1.0,      3.0,     4.0 },
       t2_ylist[] = { -1.0, 1.0/6.0, 19.0/6.0,   3.0 },
       /* incorrect values */
       t2_Mstart[] = {-1.0,    -1.0,     -1.0,    -1.0 },
       /* correct values */
       t2_Mlist[] = {  0.0,     1.0,     -2.0,     0.0 };
spline t2_dat;
t2_dat.length = 4;         t2_dat.xlist = t2_xlist;
t2_dat.ylist = t2_ylist;   t2_dat.Mlist = t2_Mstart;
spline_make_natural(&t2_dat);
for ( i = 0; i < 4; i++ )
  if ( fabs(t2_dat.Mlist[i]-t2_Mlist[i]) > 1e-12 )
  {
    printf("Error in computed Mlist[%d] = %g ", 
           i, t2_dat.Mlist[i]);
    printf("Exact value: %g\n", t2_Mlist[i]);
  }