24.3.2 describes struct iterator, which didn't exist in the
original HP STL implementation (the language wasn't rich enough at the
time). For backwards compatibility, base classes are provided which
declare the same nested typedefs:
input_iterator
output_iterator
forward_iterator
bidirectional_iterator
random_access_iterator
24.3.4 describes iterator operation distance, which takes
two iterators and returns a result. It is extended by another signature
which takes two iterators and a reference to a result. The result is
modified, and the function returns nothing.