allow use with multiple factors? (if is.list(f), construct joint factor)
more complicated designs generally?

formula interface (bpfun(x~f1*f2))?

write native CPC code??

code to check CPC output for common CPC1/warn

extensive power/coverage testing

update vignette for new (corrected) bp error/anova algorithm

test null-hypothesis simulator 

explore Phillips CPC1 rejection issue??

something hangs on doubly back-projected (= perfectly
correlated) data