Has anyone used the rlm regression function.   I am using a factor variable
as part of this regression.  If the number of occurrences of any factor is
0 or 1 it complains with the error that

'x' is singular: singular fits are not implemented in 'rlm'

The droplevels function removes 0 occurrence factors, is there a way to
eliminate ones that have occurrence of 1.

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