the generator that results from concatenating the sequences generated by g1 and g2
      
    
      Like containerOfN but with variable number of elements
      
    
      
      
    
      
      
    
      
      
    
      
      
    
      
      
    
      
      
    
      
      
    
      
      
    
      
      
    
      if gen is present then a generator that wraps in Some all the values generated by the value in gen, otherwise return a generator that always returns None
      
    
      Generates n sequences from g and concatenates them
      
    
      Generates i sequences from g, with i between n and m, and concatenates them