code: public FieldDefinition getNestedField(String fieldName) {
          if (m_type != FieldType.GROUP) {
              return null;
          }
          return m_nestedFieldMap.get(fieldName);
      }
docs: Get the {@link FieldDefinition} for the nested field with the given name from this
  group field. If this field is not a group or does not contain the specified nested
  field, null is returned.
  
  @param   fieldName   Candidate nested field name.
  @return              True if this is a group that contains the given nested field
  name.