Usefulness of explicit std::reverse_iterator(Iterator x)

[subramanian100in]

In the ISO/IEC-14882-2003 document, in section 24.4.1.1(page 526),
class template reverse_iterator is defined.

It contains the following 'explicit' ctor.
explicit reverse_iterator(Iterator x);

When is this ctor useful ?
Kindly give an example where this explicit construction of
reverse_iterator type object from an iterator type is unavoidable ?

Thanks
V.Subramanian