na.action(object, …)NA action is given.object if
NAs were handled specially, or NULL.na.action is a generic function, and na.action.default its
default method. The latter extracts the "na.action" component
of a list if present, otherwise the "na.action" attribute. When model.frame is called, it records any information
on NA handling in a "na.action" attribute. Most
model-fitting functions return this as a component of their result.options("na.action"), na.omit,
na.fail, also for na.exclude, na.pass.