Row-wise Deletion na.action
Does row-wise deletion as na.omit, but adds frequency of missing values
for each predictor
to the "na.action" attribute of the returned model frame.
Optionally stores further details if options(na.detail.response=TRUE). 
na.delete(frame)
frame | 
 a model frame  | 
a model frame with rows deleted and the "na.action" attribute added.
Frank Harrell
Department of Biostatistics
Vanderbilt University
fh@fharrell.com
# options(na.action="na.delete") # ols(y ~ x)
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