// meta-transform function that understands Transformable and List
// if f returns null, go through structure
sO lambdaMapLike transform_understandsTransformableAndList_keepUnknown(IF1 f, O o) {
  if (o == null) null;
  ping();
  try object f.get(o);
  
  IF1 myself = x -> transform_understandsTransformableAndList_keepUnknown(f, x);
  if (o cast Transformable)
    ret o.transformUsing(myself);
  if (o cast L)
    ret map(myself, o);
    
  ret o;
}

Began life as a copy of #1027750