> # Non S4
> compare(new("A", x = "1"), 1)
`x` is an S4 object of class <A>
`y` is a double vector (1)

> compare(new("A", x = "1"), globalenv())
`x` is an S4 object of class <A>
`y` is an environment

> compare(new("A", x = "1"), factor("x"))
`x` is an S4 object of class <A>
`y` is an S3 object of class <factor>

> # S4
> compare(new("A", x = "1"), new("A", x = "1"))
v No differences

> compare(new("A", x = "1"), new("A", x = "2"))
`x@x`: "1"
`y@x`: "2"

> compare(new("A", x = "1"), new("B", x = "1"))
`is(x)`:     "A"
`is(y)`: "B" "A"

