  In deeply virtual exclusive electroproduction to leading order accuracy one
accesses generalized parton distributions on their cross-over trajectory.
Combining Lorentz covariance and analyticity leads to a family of GPD sum
rules, guiding us to phenomenological concepts. As an example, we discuss the
constraints from the JLAB/Hall A data on the GPD $E$. Its first Mellin moment
is the anomalous gravitomagnetic moment, which is the unknown contribution to
the quark angular momentum.
