code: @SuppressWarnings("unchecked")
    public <X, Y, Z> Match transform(String field, BiFunction<X, Y, Z> transformer) {
      transforms.put(field, (BiFunction<Object, Object, Object>) transformer);
      return this;
    }
docs: Mark a field for transformation during serialization.
  
  @param field       The fields to include
  @param transformer The function to transform the field. Will be provided with the whole object and the field.
  @param <X>         The object being serialized
  @param <Y>         The field being serialized
  @param <Z>         The value of the field to serialize
  @return Match