  For general exact repair regenerating codes, the optimal trade-offs between
storage size and repair bandwith remain undetermined. Various outer bounds and
partial results have been proposed. Using a simple chain rule argument we
identify nonnegative differences between the functional repair and the exact
repair outer bounds. One of the differences is then bounded from below by the
repair data of a shortened subcode. Our main result is a new outer bound for an
exact repair regenerating code in terms of its shortened subcodes. In general
the new outer bound is implicit and depends on the choice of shortened
subcodes. For the linear case we obtain explicit bounds.
