if ([hFL:m] <= [h:m]) {
  return M_PI/4*[D:m]*[D:m]*[hFL:m];
} else {
  $h = [hFL:m]-[h:m];
  return M_PI/4*[D:m]*[D:m]*[hFL:m]+M_PI/3*$h*(3/4*[D:m]*[D:m]-$h*$h);
}