# NOT RUN { a <- gl(2, 4, 8) b <- gl(2, 2, 8, labels = c("ctrl", "treat")) s <- gl(2, 1, 8, labels = c("M", "F")) interaction(a, b) interaction(a, b, s, sep = ":") stopifnot(identical(a:s, interaction(a, s, sep = ":", lex.order = TRUE)), identical(a:s:b, interaction(a, s, b, sep = ":", lex.order = TRUE))) # }
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