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Gavin Wood's Yellow paper describes the a transaction receipt (p5, sec 4.3.1):
The transaction receipt is a tuple of four items com- prising the post-transaction state, R_sigma, the cumulative gas used in the block containing the transaction receipt as of immediately after the transaction has happened, Ru, the set of logs created through execution of the transaction, Rl and the Bloom filter composed from information in those logs, Rb:
R = (R_sigma;Ru;Rb;Rl)
Why is a Bloom filter used here for Rb?