I think you confused the "n-f" fraction syntax (used on the primref and in the post title) for the proposed new primitive name (with f as a prefix). If this is not the case, can you tell me what the f is supposed to stand for?
If it is the case, then I would propose either an r prefix (for reversed) or an i prefix (for inverted). We could also use a t prefix (for truncated, like used on tdis), because the proposed top is truncated in a similar fashion to tdis. However, since we start from the opposite end of the existing cyls/cyls2 primitives, perhaps t would just be confusing.
If it is the case, then I would propose either an r prefix (for reversed) or an i prefix (for inverted). We could also use a t prefix (for truncated, like used on tdis), because the proposed top is truncated in a similar fashion to tdis. However, since we start from the opposite end of the existing cyls/cyls2 primitives, perhaps t would just be confusing.