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
(M10M20...) [(A11+A21+...)(M11M21...)] + [(A12+A22+...)...] +... .
Each component may have one or more parameters that can be varied during the fit (see the newpar command write-up).