@Override
    public String asJson() {
      if (this.asJson == null) {
        this.asJson = Struct.super.asJson();
      }
      return asJson;
    }
Returns a new list where each element of this list has been expanded to the list of elements returned by
{@code mapper}.
<p>
Individual items are treated as a list containing just that item.
planetiler-core/src/main/java/com/onthegomap/planetiler/reader/Struct.java