code: @Nullable
    public final NATIVE_TYPE getDocValue (@Nonnull final Document aDoc)
    {
      final IndexableField aField = getDocField (aDoc);
      if (aField != null)
        return getFieldNativeValue (aField);
      return null;
    }
docs: Get the value of this field in the provided document
  
  @param aDoc
  The Lucene result document
  @return <code>null</code> if no such field is present, the stored value
  otherwise.