## tests of offsets, and prediction from them.

load("anorexia.rda") # copied from package MASS

## via formula
fit1 <- lm(Postwt ~ Prewt + Treat + offset(Prewt), data = anorexia)
summary(fit1)
pred <- fitted(fit1)
stopifnot(all.equal(predict(fit1, anorexia), pred))

## via argument
fit2 <- lm(Postwt ~ Prewt + Treat, data = anorexia, offset=Prewt)
summary(fit2)
stopifnot(all.equal(predict(fit2, anorexia), pred))

## now split into two parts
anorexia$o1 <- 0.9*anorexia$Prewt
anorexia$o2 <- 0.1*anorexia$Prewt
fit3 <- lm(Postwt ~ Prewt + Treat + offset(o1) + offset(o2), data = anorexia)
summary(fit3)
stopifnot(all.equal(predict(fit3, anorexia), pred))

fit4 <- lm(Postwt ~ Prewt + Treat + offset(o1), data = anorexia, offset = o2)
summary(fit4)
stopifnot(all.equal(predict(fit4, anorexia), pred))

## using more than one offset failed in R 2.8.1


## Using variable a name starting with "(weights)" :
anoD <- within(anorexia, `(weights)_2` <- Prewt)
fit5 <- lm(Postwt ~ `(weights)_2` + Treat + offset(o1) + offset(o2), data = anoD)
summary(fit5)
stopifnot(exprs = {
    all.equal(predict(fit5, anoD), pred)
    all.equal(sigma(fit5), sigma(fit1))
    is.null(model.weights(model.frame(fit5)))
})
## these were all (even the model fit!) wrong in R <= 4.2.1