R Under development (unstable) (2024-06-18 r86781 ucrt) -- "Unsuffered Consequences"
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> if (.Platform$OS.type != "windows") {
+     library(lme4)
+     library(testthat)
+     fm1 <- lmer(Reaction ~ Days + (Days|Subject), sleepstudy)
+     expect_equal(colnames(model.frame(fm1)),c("Reaction","Days","Subject"))
+     expect_equal(colnames(model.frame(fm1,fixed.only=TRUE)),c("Reaction","Days"))
+     expect_equal(formula(fm1),Reaction ~ Days + (Days | Subject))
+     expect_equal(formula(fm1,fixed.only=TRUE),Reaction ~ Days)
+ 
+     ## ugly example: model frame with compound elements
+     fm2 <- lmer(log(Reaction) ~ splines::ns(Days,3) +
+                     + I(1+Days^3) + (Days|Subject), sleepstudy)
+     expect_equal(names(model.frame(fm2)),
+                  c("log(Reaction)", "splines::ns(Days, 3)",
+                    "I(1 + Days^3)", "Days", "Subject"))
+     expect_equal(names(model.frame(fm2,fixed.only=TRUE)),
+                  c("log(Reaction)", "splines::ns(Days, 3)",
+                    "I(1 + Days^3)"))
+ 
+ } ## skip on windows (for speed)
> 
> proc.time()
   user  system elapsed 
   0.09    0.06    0.14