How to efficiently modify all values of a SortedDict?

With normal Dict I can

b=Dict(3=>4)
map!(v->v*2, values(b))

b
Dict{Int64, Int64} with 1 entry:
  3 => 8

With SortedDict I can't

a=SortedDict(2=>3)
map!(v->v*2, values(a))

ERROR: MethodError: no method matching map!(::var"#7#8", ::DataStructures.SDMValIteration{SortedDict{Int64, Int64, Base.Order.ForwardOrdering}})
Closest candidates are:
  map!(::Any, ::Base.ValueIterator{<:Dict}) at dict.jl:736
  map!(::Any, ::Base.ValueIterator{<:WeakKeyDict}) at weakkeydict.jl:140
  map!(::Tf, ::SparseArrays.AbstractSparseMatrixCSC, ::Union{LinearAlgebra.Bidiagonal, LinearAlgebra.Diagonal, LinearAlgebra.LowerTriangular, LinearAlgebra.SymTridiagonal, LinearAlgebra.Tridiagonal, LinearAlgebra.UnitLowerTriangular, LinearAlgebra.UnitUpperTriangular, LinearAlgebra.UpperTriangular, SparseArrays.SparseMatrixCSC}, ::Union{LinearAlgebra.Bidiagonal, LinearAlgebra.Diagonal, LinearAlgebra.LowerTriangular, LinearAlgebra.SymTridiagonal, LinearAlgebra.Tridiagonal, LinearAlgebra.UnitLowerTriangular, LinearAlgebra.UnitUpperTriangular, LinearAlgebra.UpperTriangular, SparseArrays.SparseMatrixCSC}...) where {Tf, N} at /opt/julia/julia-1.8.0-beta3/share/julia/stdlib/v1.8/SparseArrays/src/higherorderfns.jl:1162

I can do

for (k,v) in a
       a[k]*=2
       end

but it would be less efficient

thanks

Start by examining SortedDict, its values(a) iterator, and the existing map! behavior for Dict values. Compare how value updates are handled in the supplied loop and determine what support is needed for efficient in-place mapping. Done means the shown map!(v -> v*2, values(a)) example works for SortedDict.