code: function createLogger(instance){
  
      // generator
      function log(level){
          return function(...args){
  
              // logging backend available ?
              if (_loggingBackends.length > 0){
  
                  // trigger backends
                  _loggingBackends.forEach(function(backend){
                      backend.apply(backend, [instance, level, args]);
                  });
  
              // use default backend
              }else{
                  _defaultBackend.apply(_defaultBackend, [instance, level, args]);
              }
          }
      }
      
      return exposeAPI(log);
  }
docs: generator to create the logging instance