```
test_that("back_select works", {
set.seed(123)
x = rnorm(10)
x2 = rnorm(10)
y = rnorm(10)
df = data.frame(y, x, x2)
expect_equal(back_select(y~x*x2, data = df)$removed_vars, c("x", "x:x2"))
expect_equal(back_select(y~x, data = df)$removed_vars, c("x"))
expect_error(back_select(y+x ~ x2,data = df), regexp = NULL)
expect_error(back_select(y ~ 1,data = df), regexp = NULL)
})
```

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