Tim Bergsma wrote:
> Is there a conventional way to test for nested factors?  I.e., if 'a' 
> and 'b' are lists of same-length factors, does each level specified by 
> 'a' correspond to exactly one level specified by 'b'?

all( tapply(b, a, function(x) length(unique(x))==1 ))


J. R. M. Hosking

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