This may occur in 19th century books with signatures. Often you will have to infer the signing pattern from the signatures that are signed, put brackets around any inferred signature. When inferring signatures at the beginning, separate the inferred signature from the notated signatures. For any accompanying notes relating to this formula, the brackets do not need to be reproduced once the signature has been inferred. In the case of 2 unsigned gatherings use baseline number in front of the symbols as you would any other signature.
Bibliographies can often provide information about cancels.
If both the cancelladum and the cancellans are in the book, include a note. If a gathering has an odd number of leaves, this may mean that a blank page was removed.