code: @Nonnull
      public Collection<F> presentFields() {
             return Arrays.stream(descriptor().getFields())
                          .filter(this::isSet)
                          .collect(Collectors.toList());
      }
docs: Get a Collection of F with fields set on the builder. A.k.a is
  present.
  
  Unusual naming because it avoids conflict with generated methods.
  
  @return Collection of F