Frames of each axis
product_frames(x)A ScaleProduct.
A named list of character vectors, each coarsest-first.
product_frames(scale_product(a = scale_example(), b = scale_example2()))
#> $a
#> [1] "sector" "class" "group" "unit"
#>
#> $b
#> [1] "era" "period"
#>