More complex cases will fail

Some known cases and **workaround**:

## When a let binding LHS symbol is a denoting mutable shared variable.
```julia
@gg test() = quote
function (x)
let x = x
y -> begin
x = x + 1
x
end
end
end
end
```
**workaround**: currently, do not mutate a free variable, or box it in `Ref` or `Core.Box`:

```julia
@gg test() = quote
function (x)
let x = Ref(x)
y -> begin
x[] = x[] + 1
end
end
end
end
```

## Generators

```julia
@gg test() = quote
seq -> :(x+1 for x in seq)
end
```

**workaround**: do not use generator syntax, try to abstract it outside:

```julia
@inline gen(seq) = (x + 1 for x in seq)
@gg test() = quote
gen(seq)
end
```