code: public Optional<Metadata> getRefinedBy(Metadata meta)
    {
      return Optional.fromNullable(refines.get(meta));
    }
docs: Returns an optional metadata expression refined by the given metadata
  expression.
  
  @param meta
  a metadata expression
  @return {@link Optional#absent()} if the given metadata expression does not
  refine another metadata expression or a
  {@link Optional#of(Metadata)} containing the refined metadata
  expression