  We show how to extend systematically the FONLL scheme for inclusion of heavy
quark mass effects in DIS to account for the possible effects of an intrinsic
charm component in the nucleon. We show that when there is no intrinsic charm,
FONLL is equivalent to S-ACOT to any order in perturbation theory, while when
an intrinsic charm component is included FONLL is identical to ACOT, again to
all orders in perturbation theory. We discuss in detail the inclusion of top
and bottom quarks to construct a variable flavour number scheme, and give
explicit expressions for the construction of the structure functions $F^c_2$,
$F^c_L$ and $F^c_3$ to NNLO.
