Examples:

Assume that M1,...,Mn are multiplicative models and A1,...,An are additive models.

XSPEC> model A1          ! The single component A1 is the model

XSPEC> model A1 A2       ! model = A1 + A2

XSPEC> model ,,,M1       ! model = (A1+A2)M1

XSPEC> model M2/         ! model = M2[A2M1]

XSPEC> model ,,,,a1      ! model = M2[A2M1]+A1

XSPEC> model a1,a1,m1    ! model = (A1+A1)M1   Note that though
                           first and second components are the
                           same form, their parameters are varied
                           separately.

XSPEC> mo A1,null,A2,M1  ! model = A1 + [A2M1]

XSPEC> mo M1(A1+M2(A2))  ! model = M1[A1+M2A2]

XSPEC> mo ,,,,,B1/b      ! model is as above but with B1 included as
                           a background model.