Earlier, in section ref(ITERATORS) it was stated that iterators support
pointer arithmetic for containers storing their elements consecutively in
memory. This is not completely true: to determine the number of elements
between the elements to which two iterators refer the iterator must 
support the subtraction operator.  

Using pointer arithmetic to compute the number of elements between two
iterators in, e.g., a tt(std::list) or tt(std::unordered_map) is not possible,
as these containers do not store their elements consecutively in memory.

The function tt(std::distance)hi(distance) fills in that little gap:
tt(std::distance) expects two InputIterators and returns the number of
elements between them.

Before using tt(distance) the tthi(iterator) header file must be included.

If the iterator specified as first argument exceeds the iterator specified as
its second argument then the number of elements is non-positive, otherwise it
is non-negative. If the number of elements cannot be determined (e.g., the
iterators do not refer to elements in the same container), then tt(distance's)
return value is undefined.

Example:
        verb(    #include <iostream>
    #include <unordered_map>
        
    using namespace std;
    
    int main()
    {
        unordered_map<int, int> myMap = {{1, 2}, {3, 5}, {-8, 12}};
    
        cout << distance(++myMap.begin(), myMap.end()) << '\n'; // shows: 2
    })

The tt(iterator) header file also defines the function tt(std::size)hi(size),
returning the number of elements in a containers (as returned by the
container's tt(size) member) or of an array whose dimension is known to the
compiler at the point of tt(std::size's) call. E.g., if the size of an array
tt(data) is known to the compiler, then to call a function tt(handler)
(expecting the address of the first element of an array and the address of the
location just beyond that array) the following statement can be used:
        verb(    handler(data, data + std::size(data));)

As noted, the tt(std::size) function is defined in the tt(iterator)
header. However, it's also guaranteed available when including the header file
of a container supporting iterators (including the tt(string) header file).
