The STL map class is used for the storage and retrieval of data from
a collection in which the each element is a pair that has both a 
data value and a sort key. The value of the key is unique and is 
used to order the data is automatically. The value of an element in 
a map, but not its associated key value, may be changed directly. 
Instead, key values associated with old elements must be deleted and 
new key values associated with new elements inserted.