Vector iterator problem

    I guess that Microsoft's vector code is not good because iterator is
less flexible. If you want to use operator<= or operator>, then
assert is triggered.
    I suppose to use operator> if I don't want to use reverse_iterator.
    Why do vector documentation tell to use only operator!= or operator<?
    I wish Microsoft should rewrite their vector code. All iterators
must always use signed integer instead of unsigned integer.

#include <vector>
using namespace std;

int main()
{
    vector< int > a;
    a.push_back( 1 );
    a.push_back( 2 );
    a.push_back( 3 );
    a.push_back( 4 );

    vector< int >::iterator B = a.begin();
    vector< int >::iterator E = a.end();
    vector< int >::iterator I;

    int _v;

    // OK
    for( I = B; I != E; ++I )
        _v = *I;

    // OK
    for( I = B; I < E; ++I )
        _v = *I;

    // ERROR
    for( I = B; I <= E; ++I )
        _v = *I;
/* operator<= should call operator< automatically */

    // ERROR
    for( I = E; I != B; --I )
        _v = *I;

    // ERROR
    for( I = E; I > B; --I )
        _v = *I;

    return 0;
}