24.3.4:
"template<class InputIterator>
typename iterator_traits<InputIterator>::difference_type
distance(InputIterator first, InputIterator last);
Effects: Returns the number of increments *or decrements* needed to
get from first to last.
Requires: last must be reachable from first."

(Emphasis mine)
The requires clause implies that after a finite number of applications
of ++first, first==last must be true. Then why does the effects clause
confusingly(IMHO, at least) say "increments or decrements". Why not
just say "increments"? An input iterator that is not a bidirectional
iterator doesn't even support decrementing.


"template <class InputIterator, class Distance>
void advance(InputIterator& i, Distance n);
Requires: n may be negative only for random access and bidirectional
iterators.
Effects: Increments (or decrements for negative n) iterator reference
i by n."

IMHO, the effects clause seems inaccurate. Let us analyze it
carefully. It is equivalent to:

"For n>=0, increments the iterator reference i by n. For n<0,
decrements the iterator reference by n."

Agreed upto here?
Now, "decrements the iterator reference by n" is eqivalent to
"increments the iterator reference by (-n)". (mathematically, i-n is
equivalent to i+(-n)). For n<0, (-n) is +ve, so this is not what we
want.

It could have been phrased like this:
"For n>=0, increments the iterator reference i by n.
 For n<0, decrements the iterator reference i by -n,"

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
To iterate is human, to recurse divine.
-L. Peter Deutsch
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

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