  The proton-polarizability contribution to the muonic-hydrogen Lamb shift is a
major source of theoretical uncertainty in the extraction of the proton charge
radius. An empirical evaluation of this effect, based on the proton structure
functions, requires a systematically improvable calculation of the "subtraction
function", possibly using lattice QCD. We consider a different subtraction
point, with the aim of accessing the subtraction function directly in lattice
calculations. A useful feature of this subtraction point is that the
corresponding contribution of the structure functions to the Lamb shift is
suppressed. The whole effect is dominated by the subtraction contribution,
calculable on the lattice.
