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6 Command file: Specifying the terms in the mixed model 906.5 Interactions and conditional factorsInteractionsNew• interactions are formed by joining two or more terms with a ‘.’ or a ‘:’, forexample, a.b is the interaction of factors a and b,• interaction levels are arranged with the levels of the second factor nested withinthe levels of the first,• labels of factors including interactions are restricted to 31 characters of whichonly the first 20 are ever displayed. Thus for interaction terms it is oftennecessary to shorten the names of the component factors in a systematic way,for example, if Time and Treatment are defined in this order, the interactionbetween Time and Treatment could be specified in the model as Time.Treat;remember that the first match is taken so that if the label of each field beginswith a different letter, the first letter is sufficient to identify the term,• interactions can involve model functions,• * indicates factorial expansion (up to 5 way)a*b is expanded to a b a.ba*b*c*d is expanded toa b c d a.b a.c a.d b.c b.d c.d a.b.c a.b.d a.c.d b.c.d a.b.c.d• / indicates nested expansiona/b is expanded to a a.b• a.(b c d) e is expanded to a.b a.c a.d e. This syntax is detected by thestring ‘.(’ and the closing parenthesis must occur on the same line and beforeany comma indicating continuation. Any number of terms may be enclosed.Each may have ‘-’ prepended to suppress it from the model. Each enclosedterm may have initial values and qualifiers following. For example,yield∼site site.(lin(row) !r variety),at(site,1).(row .3 col .2)expands toyield∼site site.lin(row) !r site.variety,at(site,1).row .3 at(site,1).col .2