class Foo
{
    // only internals:
    typedef std::list<Bar> List;
    List something
    vector<List::iterator> chunks;
public:
// ...
};

auto myfind(Range, T)(Range r, T t)
{
    size_t n = 0;
    auto left = r.save;
    while(!r.empty && r.front != t) {
        r.popFront();
        n++;
    }
    return tuple(left.takeExactly(n), r);
}
// versus
template<typename I, typename T>
I find(I first, I last, const T& x)
{
    while(first != last && *first != x)
        ++first;
    return first;
}

Only truly useful thing is to decompose your program into clear subroutines, clear units, which you understand. That is the only thing I know which works.

ARK>(Ну, останется 0,001% программ, где будет присутствовать циклы, доказать завершимость которых не удастся, такие программы напишут на Ц )

auto myfind(Range, T)(Range r, T t)
{
    size_t n = 0;
    auto left = r.save;
    while(!r.empty && r.front != t) {
        r.popFront();
        n++;
    }
    return tuple(left.takeExactly(n), r);
}

void popFront() { _input.popFront(); --_n; }

_>auto d=[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], i=d.countUntil(5);
_>i-=d[0..i].retro().countUntil!((e, ref c)=>(e&1)==0&&--c==0)(2);
_>i+=d[i..$].countUntil!((e, ref c)=>(e&1)==1&&--c==0)(5);
_>writeln(d[i]);
_>

_>auto d=[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], i=d.countUntil(5);
_>i-=d[0..i].retro().countIndex!"a&1"(2);
_>i+=d[i..$].countIndex!"(a&1)==1"(5);    
_>writeln(d[i]);
_>

template<class It, class Val>
It f25(It begin, It end, Val val)
{
  auto found = find( begin, end, val );
  auto even2 = prev(make_filter_iterator( is_even, found       ), 2);
  auto odd5  = next(make_filter_iterator( is_odd, even2.base() ), 4);
  return odd5.base();
}

template<class It, class Val>
It f25(It begin, It end, Val val)
{
  auto found = find( begin, end, val );
  auto back_even2 = next(make_filter_iterator( is_even, make_reverse_iterator(found) ), 1);
  auto odd5       = next(make_filter_iterator( is_odd,  back_even.base().base()      ), 4);
  return odd5.base();
}