@progress with multi-variable for loop

[yha]

@progress doesn't work when later iterables refer to previous iteration variables:

julia> x = [1:3, 1:2, 5:6];

julia> @progress for y in x, z in y
           @show z
       end
ERROR: UndefVarError: y not defined

I realize there's a problem computing the progress in this case, but perhaps there could be a better error message or a fallback to compute progress only based on the outer loop.