You could possibly try to explain away the behavior for a missing main
effects term, since without the main effects term we don't have main effect
columns in the model matrix used to compute the interaction columns (At
best this is undocumented behavior--I still think it's a bug, as we know
how we
In R devel, function 'factor' has been changed, allowing and merging duplicated
'labels'.
Issue 1: Handling of specified 'labels' without duplicates is slower than
before.
Example:
x <- rep(1:26, 4)
system.time(factor(x, levels=1:26, labels=letters))
Function 'factor' is already rather slow
