One variant of these protocols permits reuse of Q at the expense of
allowing the bank to link repeated uses of Q: instead of blind
signing h(Q), the bank blind signs h(Q,s), where s is some
serial number chosen by the consumer. In this fashion, many tokens
( 
) can all specify the same key (Q). Only the
bank sees Q (in 4), and so only the bank can link repeated
uses of the same key to each other.