Exercise 2.60: We specified that a set would be
represented as a list with no duplicates.  Now suppose we allow duplicates.
For instance, the set 
  
    {
    1
    ,
    2
    ,
    3
    }
  
 could be represented as the list (2 3 2 1
3 2 2).  Design procedures element-of-set?, adjoin-set,
union-set, and intersection-set that operate on this
representation.  How does the efficiency of each compare with the corresponding
procedure for the non-duplicate representation?  Are there applications for
which you would use this representation in preference to the non-duplicate one?