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casued by this code

function compose_cdm_specimen_or_observation_tree_entry() {

...


                    /* ---- SpecimenOrObservationBase --- */
                    case 'derivationEvent':
                        @_description_list_group_add($groups, cdm_occurrence_field_name_label($field), $value, NULL, 1);  // <--------- HERE $value is object
                        break;
                    case 'kindOfUnit':
                        @_description_list_group_add($groups, cdm_occurrence_field_name_label($field), $value, NULL, 1);
                        break;