// if f returns null, go through structure
sO transform_transformableAndList(IF1 f, O o) {
  if (o == null) null;
  ping();
  try object f.get(o);
  
  if (o cast Transformable)
    ret o.transformUsing(x -> transform(f, x));
  
  if (o cast L)
    ret map(x -> transform_transformableAndList(f, x), o);
    
  fail("Don't know how to transform: " + className(o));
}