This is a quote from the book "On Zero Confirmation Transactions" by Chris Pacia
...�� policy, technically there is nothing forcing them to do so. In theory, a miner could patch their code to implement some other policy and in practice some have.

Consider the case
of the Eligius mining pool. The pool operator believes that certain types of transactions, such as gambling transactions, constitute “spam” and should be banned from the network. So he runs a patch that checks each transaction for payments to certain addresses and refuses to include those transactions in his pool’s blocks. His code refers to it as an “is notorious” check. And...
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