stl - What is multi-pass guarantee per C++ ISO standard? -

reading working draft n3337-1, standard programming language c++, 24.2.5 forward iterators, page 806.

from draft:

two dereferenceable iterators a , b of type x offer multi-pass guarantee if:
— a == b implies ++a == ++b and
— x pointer type or expression (void)++x(a), *a equivalent expression *a.

[ note: requirement a == b implies ++a == ++b (which not true input , output iterators) , removal of restrictions on number of assignments through mutable iterator (which applies output iterators) allows use of multi-pass one-directional algorithms forward iterators. —end note ]

could re-interpret in easier terms ? understand forward iterators multi-pass, don't understand how accomplished per c++ standard requirements.

the terms states all, i'd think: can pass through sequence multiple times , remember positions within sequence. long sequence doesn't change, starting @ specific position (iterator) you'll traverse on same objects want in same order. however, can go forward, there no way move backwards. canonical example of sequence singly-linked list.

the quoted clause says, if have 2 iterators comparing equal , increment each 1 of them, same position , compare equal again:

if (it1 == it2) {     ++it1;     ++it2;     assert(it1 == it2); // has hold multi-pass sequences } 

the weird expression ++x(a), *a intended advance iterator independent a , requirement ++x(a), *a being equivalent *a means iterator on sequence using independent iterator doesn't change a refers to. unlike input iterator ++init(a), *a not equivalent *a first expression can have change position, possibly invalidating a and/or change value referring to.

by contrast, single-pass sequence (input , output iterations in standard terms) can traversed once: trying traverse sequence multiple times not work work. canonical example of sequences input keyboard , output console: once read, can't same characters again , once sent can't undo characters.