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Combines two T instances associatively.
result of combining l and r
 
      
    
       
      
    
      Returns an instance of T calculated by summing all instances in
iter in one pass.
Returns an instance of T calculated by summing all instances in
iter in one pass. Returns None if iter is empty, else
Some[T].
instances of T to be combined
None if iter is empty, else an option value containing the summed T
 
      
    
       
      
    
      
     
      
    
       
      
    
       
      
    
       
      
    
      Returns the identity element of T for plus.
Returns the identity element of T for plus.