  The FONLL general-mass variable-flavour number scheme provides a framework
for the matching of a calculation in which a heavy quark is treated as a
massless parton to one in which the mass dependence is retained throughout. We
describe how the usual formulation of FONLL can be extended in such a way that
the heavy quark parton distribution functions are freely parameterized at some
initial scale, rather than being generated entirely perturbatively. We
specifically consider the case of deep-inelastic scattering, in view of
applications to PDF determination, and the possible impact of a fitted charm
quark distribution on $F_2^c$ is assessed.
