[Rd] `body<-` removes the class attributes

[Antoine Fabri]

Dear all,

`body<-` removes the class of the input function, it's undocumented and I
not what I would expect.

``` r
foo <- function() {1}
class(foo) <- "bar"
attr(foo, "class")
#> [1] "bar"
body(foo) <- 2
attr(foo, "class")
#> NULL
```

Best regards,

Antoine

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