Extract information on the NA action used to create an object.
any object whose
further arguments special methods could require.
na.action is a generic function, and
default method. The latter extracts the
of a list if present, otherwise the
model.frame is called, it records any information
NA handling in a
"na.action" attribute. Most
model-fitting functions return this as a component of their result.
Information from the action which was applied to
NAs were handled specially, or
Chambers, J. M. and Hastie, T. J. (1992) Statistical Models in S. Wadsworth & Brooks/Cole.
na.fail, also for