> compare(quote(a), quote(b))
`x` is `a`
`y` is `b`

> compare(quote(a + b), quote(b + c))
`x`: "a + b"
`y`: "b + c"

> x <- y <- quote(foo(1:3))
> y[[2]] <- 1:3
> compare(x, y)
`deparse(x)` equals `deparse(y)`, but AST non-identical

