code: function step() {
          // Back to the first iterator.
          if (index === count - 1) {
              index = -1;
          }
  
          // Go to the next iterator.
          index++;
  
          // Ignore empty iterators.
          while(empties.has(index)) {
              if (index === count - 1) {
                  index = -1;
              }
  
              index++;
          }
  
          return iterators[index];
      }
docs: Returns next iterator.
  
  Returns the first iterator after the last one.
  
  @returns {Iterator}