In biblatex, the field
volumes can be used to indicate the number of volumes in a multi-volume work. But how can I indicate that a multi-volume work has e.g. 8 numbered volumes plus a supplementary unnumbered volume, e.g. an index?
Ideally, I would like to output a reference with a format like "8 vols, 1 index", "8 vols, 1 appendix " or "8 vols, 1 supplementary vol."