  We analyze the properties of the ACOT scheme for heavy quark production and
make use of the \MSbar massless results at NNLO and N3LO for the structure
functions F2 and FL in neutral current deep-inelastic scattering to estimate
the higher order corrections. For this purpose we decouple the heavy quark mass
entering the phase space from the one entering the dynamics of the short
distance cross section. We show numerically that the phase space mass is
generally more important. Therefore, the dominant heavy quark mass effects at
higher orders can be taken into account using the massless Wilson coefficients
together with an appropriate slow-rescaling prescription implementing the phase
space constraints. Combining the exact ACOT scheme at NLO with these
expressions should provide a good approximation to the missing full calculation
in the ACOT scheme at NNLO and N3LO.
