set

Sets are containers that store unique elements following a sorted order.
* Sets implement binary search tree. Sets store the elements in sorted order.
* All the elements in a set have unique values.  The value of an element also is the key to identify it. The values in a set are unindexed. The values are immutable.The value of the element cannot be modified once it is added to the set, though it is possible to remove and then add the modified value.