A sequence database is just a large binary file. It is not a true database. As such, if the sequence removed is not physically the last sequence within the file, it is not removed, but instead it is erased.

If the sequence was in a dance, then that sequence is either displayed as empty when shown via a dance (when the sequence was not the last sequence in the database), or the sequence is displayed with an error message saying that the sequence was not found.

A dance is just a set of links to sequences. Therefore, if the sequence is deleted, the links point to an empty or non-existent sequence.