For the moment I'll call the relation "confusability". Given any two labels (in no particular order), they are either confusable or not, and it is possible to compute that boolean value. The computation might enumerate all the labels that could be confused with one of the inputs and check whether the other input is among them, or it might be possible to be more efficient by using a clever algorithm. We'll worry about that later.
tedd -- http://sperling.com/