differentiate produces unexpected results on a simple expression

Perhaps I have misunderstood the calling conventions but

``` julia
julia> differentiate(:(x/(K+x)),:K)
:(*(-1,x))
```

is not what I expected.  I was rather hoping for

``` julia
:(/(-(x),^(+(K,x),2)))
```

or, perhaps,

``` julia
:(/(-(x),abs2(+(K,x))))
```
