test_that("is_exposure_constant", {
    
    d1 = data.frame(
        conc = c(rep(1,3), rep(4,3)),
        replicate = c(rep("a",3), rep("b",3)))
    d2 = d1 ; d2$conc[1] = NA
    d3 = d1 ; d3$conc[1] = d3$conc[1] + 2
    
    expect_true(is_exposure_constant(d1))
    expect_false(is_exposure_constant(d2))
    expect_false(is_exposure_constant(d3))
    
    expect_true(is_exposure_constant(PRZ_cst))
    expect_false(is_exposure_constant(PRZ_var))

})