code: public RelationType joinWith(RelationType other)
      {
          List<Field> fields = ImmutableList.<Field>builder()
                  .addAll(this.allFields)
                  .addAll(other.allFields)
                  .build();
  
          return new RelationType(fields);
      }
docs: Creates a new tuple descriptor containing all fields from this tuple descriptor
  and all fields from the specified tuple descriptor.