code: private static String inputQualifiedNamePrefix(DMNNode input, DMNModelImpl model) {
          if (input.getModelNamespace().equals(model.getNamespace())) {
              return null;
          } else {
              Optional<String> importAlias = model.getImportAliasFor(input.getModelNamespace(), input.getModelName());
              if (!importAlias.isPresent()) {
                  MsgUtil.reportMessage(LOG,
                                        DMNMessage.Severity.ERROR,
                                        ((DMNBaseNode)input).getSource(),
                                        model,
                                        null,
                                        null,
                                        Msg.IMPORT_NOT_FOUND_FOR_NODE_MISSING_ALIAS,
                                        new QName(input.getModelNamespace(), input.getModelName()),
                                        ((DMNBaseNode)input).getSource());
                  return null;
              }
              return importAlias.get();
          }
      }
docs: DMN v1.2 specification, chapter "10.4 Execution Semantics of Decision Services"
  The qualified name of an element named E that is defined in the same decision model as S is simply E.
  Otherwise, the qualified name is I.E, where I is the name of the import element that refers to the model where E is defined.