code: public String getDoi() {
      if (Header_Type.featOkTst && ((Header_Type)jcasType).casFeat_doi == null)
        jcasType.jcas.throwFeatMissing("doi", "de.julielab.jules.types.Header");
      return jcasType.ll_cas.ll_getStringValue(addr, ((Header_Type)jcasType).casFeatCode_doi);}
docs: getter for doi - gets document object identifier
  @generated
  @return value of the feature