Changing order of Type Parameters (0.6)

General Usage
1

Why is B ok and BB not? How do I get BB to work?

abstract type AbsA{F<:AbstractFloat} end
struct A{F} <: AbsA{F} 
    data::F
end
a = A(9.) # OK

abstract type AbsB{F<:AbstractFloat} end
struct B{F, TA<:AbsA{F}} <: AbsB{F}
    x::TA
    y::F
end
b = B(A(9.), 1.) # OK

struct BB{TA<:AbsA{F}, F} <: AbsB{F} 
    x::TA
    y::F
end
# Throws error UndefVarError: F not defined
2 
julia> struct BB{F, TA<:AbsA{F}} <: AbsB{F} 
           x::TA
           y::F
       end