Helper functions for modelsum

A set of helper functions for modelsum.

is.modelsum(x)

is.summary.modelsum(x)

na.modelsum(object, ...)

Arguments

x	A `modelsum` object.
object	A `data.frame` resulting from evaluating a `modelsum` formula.
...	Other arguments, or a vector of indices for extracting.

na.modelsum returns a subsetted version of object (with attributes).