>
> #= This is the function from Base that is being used when you call & on
>> two sparse matrices
>> function ($f){S,T}(A::AbstractArray{S}, B::AbstractArray{T})
>> F = similar(A, promote_op($F,S,T), promote_shape(size(A),size(B)))
>> for i in eachindex(A,B)
>> @inbounds F[i] = ($f)(A[i], B[i])
>> end
>> return F
>> end
>>
>> # look at what eachindex does for SparseMatrixCSC
>> a = sprand(10,10,0.3)
>> @show collect(eachindex(a))
>> 100-element Array{CartesianIndex{2},1}:
>> CartesianIndex{2}((1,1))
>> CartesianIndex{2}((2,1))
>> CartesianIndex{2}((3,1))
>> CartesianIndex{2}((4,1))
>> CartesianIndex{2}((5,1))
>> CartesianIndex{2}((6,1))
>> CartesianIndex{2}((7,1))
>> CartesianIndex{2}((8,1))
>> CartesianIndex{2}((9,1))
>> CartesianIndex{2}((10,1))
>> CartesianIndex{2}((1,2))
>> CartesianIndex{2}((2,2))
>> CartesianIndex{2}((3,2))
>> CartesianIndex{2}((4,2))
>> ⋮
>> CartesianIndex{2}((6,8))
>> CartesianIndex{2}((7,8))
>> CartesianIndex{2}((8,8))
>> CartesianIndex{2}((9,8))
>> CartesianIndex{2}((10,8))
>> CartesianIndex{2}((1,9))
>> CartesianIndex{2}((2,9))
>> CartesianIndex{2}((3,9))
>> CartesianIndex{2}((4,9))
>> CartesianIndex{2}((5,9))
>> CartesianIndex{2}((6,9))
>> CartesianIndex{2}((7,9))
>> CartesianIndex{2}((8,9))
>> CartesianIndex{2}((9,9))
>> CartesianIndex{2}((10,9))
>> CartesianIndex{2}((1,10))
>> CartesianIndex{2}((2,10))
>> CartesianIndex{2}((3,10))
>> CartesianIndex{2}((4,10))
>> CartesianIndex{2}((5,10))
>> CartesianIndex{2}((6,10))
>> CartesianIndex{2}((7,10))
>> CartesianIndex{2}((8,10))
>> CartesianIndex{2}((9,10))
>> CartesianIndex{2}((10,10))
>>
>> The problem is that the generic AbstractArray implementation of & and
some other operators is iterating over all the n x m entries of A.
