Specify Multiple Instances of a Nonlinear Term in a gnm Model Formula
A symbolic wrapper, for use in the formula argument to
gnm, to specify multiple instances of a term specified
by a function with an inst argument.
instances(term, instances = 1)
| term | a call to a function with an inst argument, which specifies some term. | 
| instances | the desired number of instances of the term. | 
A deparsed expression representing the summation of term
specified with inst = 1, inst = 2, ..., inst =
    instances, which is used to create an expanded formula.
Heather Turner
## Not run: 
## (this example can take quite a while to run)
##
## Fitting two instances of a multiplicative interaction (i.e. a
## two-component interaction)
yield.scaled <- wheat$yield * sqrt(3/1000)
treatment <- factor(paste(wheat$tillage, wheat$summerCrop, wheat$manure,
                          wheat$N, sep = ""))
bilinear2 <- gnm(yield.scaled ~ year + treatment +
                 instances(Mult(year, treatment), 2),
                 family = gaussian, data = wheat)
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