code: public static ImmutableSet<QuotedID> getShared(RAExpressionAttributes re1,
                                                     RAExpressionAttributes re2) {
  
          return re1.attributeOccurrences.keySet().stream()
                  .filter(id -> !re1.isAbsent(id) && !re2.isAbsent(id))
                  .collect(ImmutableCollectors.toSet());
      }
docs: NATURAL JOIN
  
  @param re1 a {@link RAExpressionAttributes}
  @param re2 a {@link RAExpressionAttributes}
  @return a {@link ImmutableSet}<{@link QuotedID}>