code: public boolean hasNestedField(String fieldName) {
          if (m_type != FieldType.GROUP) {
              return false;
          }
          return m_nestedFieldMap.containsKey(fieldName);
      }
docs: Indicate if this group field contains an immediated-nested field with the given
  name. If this field is not a group, false is returned.
  
  @param   fieldName   Name of candidate nested field.
  @return              True if this is a group field that contains the given name
  as an immediated nested field.