code: public Iterable<FieldDefinition> getNestedFields() {
          if (m_type != FieldType.GROUP) {
              return null;
          }
          assert m_nestedFieldMap != null;
          assert m_nestedFieldMap.size() > 0;
          return m_nestedFieldMap.values();
      }
docs: If this is a group field, return the nested {@link FieldDefinition} objects as an
  {@link Iterable} collection. If this is not a group field, null is returned.
  
  @return  Nested field definitions as an Iterable collection if this is a group
  field, otherwise null.