Cat() nested transducers

[Uroc327]

How can I concatenate nested transducers generated by Map()s?

Consider the following example code.

using Combinatorics
using Transducers

a() = partitions(10, 3) |> MapCat(unique ∘ permutations) |> collect
b() = partitions(10, 3) |> MapCat(permutations) |> Unique() |> collect
#c() = partitions(10, 3) |> MapCat(Unique() ∘ permutations) |> collect
#d() = partitions(10, 3) |> MapCat(part -> part |> permutations |> Unique()) |> collect

This generates all unique permutations of arrays of three integers summing to ten. Ideally, I would like to have the Unique() inside the MapCat because after Cat()ing all the permutations, there are a lot of elements because of which the buffer for checking uniqueness becomes larger than what fits into memory (for larger values than 10 and 3). Therefore, b() only works for small problems. On the other hand, using the unique function instead of the Unique() transducer allocates and collects, which is also suboptimal. Thus, I'd also like to avoid b().

The versions with Unique() inside the MapCat() throw a MethodError, though. Specifically,

ERROR: MethodError: no method matching unwrap(::Transducers.Reduction{Unique{…}, Transducers.Reduction{…}}, ::BangBang.SafeCollector{Empty{…}})
The function `unwrap` exists, but no method is defined for this combination of argument types.