In order that a particular multiplicative model may be applied to some,
but not all, additive models, the following syntax is used: the N components
are grouped into subgroups of additive and multiplicative models:
XSPEC> [<mult. subgroup 0>] <group1> <group2> ... <groupI>
where each group is <groupJ> =:: <add. subgroupJ> [<mul. subgroupJ> or NULL],
a string of one or more additive model components followed by an
optional list of one or more multiplicative model components. The model
named null is a special string used for punctuation to separate two
adjacent additive subgroups without an intervening multiplicative model.
Each model in a particular multiplicative subgroup is applied to the
sum of all the additive models in the immediately preceeding subgroup.
Any models in the optional multiplicative subgroup 0 are applied to the
net spectrum from the sum of all the groups. Therefore, if Aij is the
ith additive model in the jth group, and similarly, Mij is a multiplicative
model, the total spectrum is given by
(M10*M20*...) * [(A11+A21+...)*(M11*M21*...)] +
[(A12+A22+...)*...] +... .
Each component may have one or more parameters that can be varied during the
fit (see the newpar command write-up).