Inclusion between set unions

[mforets]

a = UnionSetArray([rand(HPolygon) for _ in 1:3])
b = UnionSetArray([rand(HPolygon) for _ in 1:3]);
a ⊆ b

MethodError: no method matching length(::UnionSetArray{Float64,HPolygon{Float64}})
Closest candidates are:
  length(!Matched::Core.SimpleVector) at essentials.jl:597
  length(!Matched::Base.MethodList) at reflection.jl:819
  length(!Matched::Core.MethodTable) at reflection.jl:893
  ...

Stacktrace:
 [1] issubset(::UnionSetArray{Float64,HPolygon{Float64}}, ::UnionSetArray{Float64,HPolygon{Float64}}) at ./abstractset.jl:234
 [2] top-level scope at In[171]:3

[schillic]

This is by far the most difficult issue here. I doubt that there is a feasible solution in general. In theory I would take the pairwise intersection and check that their union is again the original set. But an equivalence check with a union seems almost as hard as the original problem. For Intervals this would work, though.

[mforets]

A sufficient condition is easy, we can do that first.