```
# NOT RUN {
x <- airquality[, -1] # x is a regression design matrix
y <- airquality[, 1] # y is the corresponding response
stopifnot(complete.cases(y) != is.na(y))
ok <- complete.cases(x, y)
sum(!ok) # how many are not "ok" ?
x <- x[ok,]
y <- y[ok]
# }
```

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