Garbage collection for objects created with `@set`

I'm having difficulty releasing the memory of objects created with `@set`.

For the following code, both arrays are expected to be garbage collected, but I always end up with one copy left in my memory if I `htop` my memory usage. 

```julia
a = rand(10000, 10000)
b = @set a[1] = 1
a = nothing
b = nothing
GC.gc(true)
```
Any thoughts? Thanks.