lockstep alternative for StoppingPolicy.longest

[Andrej Mitrovic]

I need to iterate through two arrays and do some special comparisons,
but the arrays are not guaranteed to have the same length. lockstep
doesn't work with the "longest" policy, e.g.:

int[] a = [1, 2];
int[] b = [1, 2, 3];

foreach (aa, bb; lockstep(a, b, StoppingPolicy.longest))  // throws
{
}

What I would like to have is the ability to set a sentinel value when
.empty returns true for one of the arrays, this would enable this
policy to work. For example I could have:

foreach (aa, bb; lockstep(a, b, StoppingPolicy.longest, -1))  // sentinel is -1
{
    // if aa or bb doesn't exist it's set to -1
}

or alternatively aa/bb could be pointers, and the sentinel would
conveniently be null. Is anything like this in Phobos already?