code: public static Schema unionOf(Iterable<Schema> schemas) {
      List<Schema> schemaList = ImmutableList.copyOf(schemas);
      Preconditions.checkArgument(schemaList.size() > 0, "No union schema provided.");
      return new Schema(Type.UNION, null, null, null, null, null, null, schemaList);
    }
docs: Creates a {@link Type#UNION UNION} {@link Schema} which represents a union of all the given schemas.
  The ordering of the schemas inside the union would be the same as the {@link Iterable#iterator()} order.
  
  @param schemas All the {@link Schema Schemas} constitutes the union.
  @return A {@link Schema} of {@link Type#UNION UNION} type.