private Struct parseJson() {
      return struct != null ? struct : (struct = of(JsonConversion.readValue(value, Object.class)));
    }
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