The $ and [[ operators get the value
of a single parameter in the model. The
[ gets several values, returning a
named list.
# S4 method for mrgmod
$(x, name)
# S4 method for mrgmod
[[(x, i, exact = TRUE)
# S4 method for mrgmod
[(x, i)mrgmod object
parameter to take
an element to select
not used