[R] Handling of NAs in functions lrm and robcov

[Christof Bigler]

Hi R-helpers

I have a dataframe DF (lets say with the variables, y, x1, x2, x3, ..., 
clust) containing relatively many NAs.
When I fit an ordinal regression model with the function lrm from the 
Design library:
model.lrm <- lrm(y ~ x1 + x2, data=DF, x=TRUE, y=TRUE)
it will by default delete missing values in the variables y, x1, x2.
Based on model.lrm, I want to apply the robust covariance estimator 
using a cluster variable:
model.robcov.lrm <- robcov(model.lrm, cluster=clust)
How can I remove observations in the cluster variable clust that 
contain NAs in y, x1, and x2?

Thanks,
Christof